At the moment, it may be more trouble than it's worth. By historical standards, today's 30-year fixed mortgage rate of just more than 7 percent is very low. It is unlikely that rates will drop dramatically in the 30 to 60 days you have to lock in a mortgage rate, as 10-year Treasury rates have already fallen substantially. A modest drop in mortgage rates wouldn't save you an awful lot, anyway. If I were shopping for a mortgage today, I'd jump at the chance to get 7 percent.
Lower rates are very rare.
Be sure to quiz prospective lenders, because advertisements and loan-listing services rarely describe all fees.