5 popular myths that cost you money

This post comes from Jim Wang at partner blog Bargaineering.

One of the fun parts about reading and writing about personal finance and money all day is that you run into a lot of good advice and a lot of bad advice. Over the years, I've been amazed at how much of both is repeated with great regularity.

Whereas some bad advice hurts no one, a lot of money myths are costing some people money (and helping others make a lot more than they should). So today I'll be hitting five popular myths that span your entire life, from spices to gasoline, with the hope that it spurs a discussion that helps us all understand why these myths are wrong and why they're costing us money.

Let the air conditioning run all day. For most HVAC systems, there is one setting -- on. The system is either running or it's not. It doesn't work harder if it needs to raise or lower the temperature more, it just works. If it's 95 degrees out and your thermostat is set to 85, it'll run until it hits 85.

When you leave your AC on all day, here's what happens most of the time. It runs until it hits 85, then it stops. When the internal temperature goes to 86, it turns on again. When it reaches 85, it shuts off. On. Off. On. Off. You get home and everything feels great but your unit has done the equivalent of about a dozen sprints in the time you were out.

Instead, get a programmable thermostat and set it to turn on half an hour before you get home. You won't be the wiser and your unit will thank you for not jerking it around all day.

• How much will my savings grow?

Spices lose potency. Herbs and spices in your cupboards do lose their potency after time -- that statement itself is not a myth. The myth is that you should replace them. Unlike vitamins and drugs, which do the same, it doesn't matter if your spices lose potency. Just use more of them. Spice companies would love for you to believe that you should replace them because spices and herbs are insanely expensive. (If you fancy yourself a chef who needs only the best, you should be using the fresh stuff anyway.)

• Try the 50-30-20 budget

Oh, and while you're at it, go to "ethnic" grocery stores (any store that isn't your neighborhood grocery store) for spices because they're a lot cheaper there.

Premium gas is better. Premium gas is not better. Premium gas is just higher octane gas, and higher octane means it can better withstand compression before igniting. Cars that require premium gas will compress the gas-air mixture to a higher psi before igniting it. If you use lower octane gas in a premium-only vehicle, you'll get a knocking. That's the gas igniting before it's supposed to. If you put premium gasoline into a car that requires only regular, you're just wasting money. Post continues after video (the gas prices you'll see are from January).

Your taxes are complicated. There are really only two reasons you should pay an accountant to do your taxes:

1. Your time is more valuable than the cost of an accountant.
2. Your taxes are very complicated.

If it's No. 1, you will find no argument from me. You get to decide how valuable your time is. I don't. Your parents don't. Your spouse doesn't. Your friends don't.

If it's No. 2, I'd argue that it's probably not as complicated as you think and, with the sophistication of software packages, you can probably do it in a few hours without messing it up. Before you pay an accountant a few hundred dollars, invest some time in a package like TurboTax. For the packages online, you pay at the end, so you can complete your return (which is mostly data entry of forms you have to collect anyway) for free.

Almost any rule of thumb. The problem with rules of thumb are that they try to apply a basic idea to everyone. Rules of thumb like you should save 10% of your income or spend only 33% of your income on housing may sound like a good idea but they’re killing peoples’ finances. My finance situation is different from yours. It's different from that of someone in college. It's different from that of someone who is retiring.

Should a retiree be saving 10% of their income? Probably not. They're spending their savings.

Should I retire at 65? Why 65? Why not later? Sooner?

The lesson here is that you should view any rule of thumb with a healthy dose of skepticism.

More on Bargaineering and MSN Money:

• Don't hesitate to report a credit card as stolen
• 4 reasons running out of gas hurts
• How to calculate your full effective tax rate
• Do programmable thermostats really save money?
• Is premium gas worth the premium price?
• Fact or fiction? 8 gas-saving myths