Determining whether it’s good or bad to buy a prepaid item depends entirely on what’s being offered. There’s no universal advice or rule of thumb. Here’s a guide to some of the common forms of prepaid offers consumers might encounter, separated into whether you should consider buying them.

Cell phones

Unless you want the capabilities of a smart phone with Internet access, a pay-as-you-go prepaid wireless phone is a great idea and can be far cheaper than a cell phone with a monthly contract, experts say. That’s especially true if you use only a few hundred minutes per month. But even moderately heavy users can reap savings, says Consumer Reports magazine.

Consumers are starting to get the message. The fourth quarter of 2009 marked the first time new prepaid wireless customers in the United States outnumbered new contract-based customers, according to the New Millennium Research Council, a Washington think tank whose work focuses on telecommunications and technology.

With a prepaid phone, you buy an inexpensive phone and load minutes onto the phone as needed. You can buy minutes online or at retail stores in the form of a prepaid phone card that has a code you type into the phone. Taxes and fees are included in the price of the minutes. A few downsides: The phones are not the most current models, and minutes expire after a set period.

Highly rated prepaid providers include Tracfone, Net10, Virgin Mobile and T-Mobile.

Some wireless providers also offer month-to-month, flat-fee prepaid plans with more full-featured phones and service.

Discounts

All sorts of services, from insurance to pest control, might offer a discount for prepaying in a lump sum upfront rather than monthly. This is generally a good idea nowadays if you can afford it, assuming you can get money back for any unused portion of the service if you discontinue it. Why? Typically, you would compare the discount with how much interest you could otherwise earn in a safe account, such as a bank account. But interest rates are so low now that prepaying for services will almost surely reap a higher return.

Credit cards

A couple of types of cards can fall under this category. One is a secured credit card where you deposit an amount, say $500, and you can charge up to that amount. This is useful for young people trying to establish a credit history or for those recovering from damaged credit and personal bankruptcy. The key is to find a card that reports to all three major credit bureaus, which will start improving your credit scores if you pay on time, said Liz Pulliam Weston, author of “Your Credit Score.” A secured credit card that converts to a regular credit card after a period is ideal, she said.

The other type is a refillable prepaid credit card that acts like a debit card. They have the advantages of credit cards, such as being able to book a rental car, without the downsides of overspending or finance charges, Weston said.

Prepaid credit cards can also be useful for parents to use as a teaching tool for older children. Prepaid cards typically do not help you build a credit history.

The downside with all these cards are the fees, Weston said. For example, a prepaid card might have a one-time $20 processing or application fee, plus a fee of $5 per month. Research cards at such comparison sites as CardRatings.com, CreditCards.com and LowCards.com.

Mortgage points

This is essentially prepaying interest to get a lower ongoing interest rate. Base your decision on how long you’ll remain in the house.

Figure out how many months of lower mortgage payments it would take to break even with the amount you paid in points. For example, if you paid $3,000 in points for a slightly lower interest rate that saves you $100 per month on your mortgage, it would take 30 months to break even. If you’re very likely to stay in the house longer, paying points could be worthwhile.

Tuition

Prepaid tuition plans have helped millions of families in the past, but consumers nowadays need to be wary, said college-savings expert Joe Hurley. That’s because states, which sponsor prepaid 529 plans, are suffering budget shortfalls and might not be able to honor pledges to pay for future college costs, he said.

About half of the original 18 states that offered prepaid plans have closed their plans to new enrollment. Of the remaining, just four plans are guaranteed — that is, backed by the full faith and credit of the state. Those states are Massachusetts, Mississippi, Texas and Washington. A fifth, in Florida, is guaranteed if the student is within five years of enrolling at college.

Also, it’s no longer “tomorrow’s tuition at today’s prices.”

“Most plans charge considerably more than today’s prices,” Hurley said. “That is their way to maintain financial solvency.”

The more common, investment-style 529 plans, where growth on investment money can be used tax-free for college expenses, could be a better option. In fact, associated built-in fees have generally come down in recent years, making them a better deal, Hurley said.

Prepaid funerals

Almost never a good idea, said Joshua Slocum, executive director of the Funeral Consumers Alliance, which attributes one-third of complaints it receives to prepaid funerals. “Consumers are far too gullible for their own good when it comes to this stuff,” he said.

If you move to a different region of the country, die while traveling or simply change your mind, you might not receive a full refund, with interest, of your prepayment. And money paid now might not cover inflated funeral costs in the future, possibly resulting in substituting cheaper merchandise or needing more money from survivors. And, of course, the seller of the funeral services could go out of business.

A better option would be simply to place the money in a bank “payable on death” account, also called a Totten trust, and name a survivor as beneficiary.

Extended warranties

Extended warranties, also called service contracts, mean you are essentially prepaying for a repair. They are a source of huge profit for retailers and seldom a good deal for consumers, consumer advocates agree. Consumer Reports, for example, has long advised shoppers to just say no.

Time-share vacations

Time-share owners are essentially prepaying for the hotel or resort condo they would have paid for during future vacations. Problem is, they’re very expensive, an average of about $20,500 in 2009. That means they take years to break even. You’re also locked into a single vacation spot or must deal with a sometimes-cumbersome system of swapping time-share locations. Time shares also come with annual maintenance fees, which averaged about $674 last year. Most important, there is almost no secondary market for time shares, meaning you’ll have trouble selling it, let alone for a good price.