Part of the fun of checking out mortgage offers online (if you can call it fun) is testing out the lenders' affordability calculators. If they're accurate, they're giving you the maximum amount you can afford based on your income and credit data. It may be fine to know the outside amount you can borrow, but is it really a financially healthy move for you to borrow the maximum? That's a critical question you must answer for yourself.

Also, if a simple online calculator requires you to supply a lot of confidential information — including your Social Security number — walk away. While most reputable sites will not ask for that information until you are ready to query a specific lender, you must know how they're going to handle that information and be comfortable offering it.