Interest rates are based on numerous economic, financial and credit based factors that adjust daily. In addition, lenders can vary on qualification criteria from program to program. If you like the rate today, the safe bet is to lock. Even if rates improve, they wouldn't improve enough in the short term to make you cry about it. But if you are an ardent market bear, and accept the risk of negative mortgage headlines, and believe the economy will just get more bad news next week, and you have the money to risk, you may benefit from floating. Just remember, it always seems more painful to have not locked when you should have as opposed to locking and then watching rates get a little better.