For would-be homeowners, choosing whether to lock in a mortgage rate with your lender or wait for rates to fall is a difficult decision. That's because even a seemingly small change in interest rates can add or subtract tens of thousands of dollars over the life of a typical 30-year mortgage.The decision is even more difficult these days with such high volatility in mortgage rates over the past month. So if your mortgage lender is asking whether you want to "lock in" or "float," what should you do? For starters, know what the two terms mean: Locking in means finalizing your mortgage rate and points on a fixed-rate or adjustable-rate mortgage (only the initial rate in the case of the latter). You'll have the same interest rate on your loan regardless of whether rates go up or down. The alternative to locking in is "floating," which means your rate will float up and down along with any daily changes in mortgage rates until you decide to lock in the rate.