Lenders protect themselves from intra-day market fluctuations by keeping that "valid through" time/date as short as possible. Know that when you do lock a rate, most lenders will now issue what's known as a "Change of Circumstance" document to you and it will indicate the change as the rate lock. You will also then get a new GFE showing the locked rate and the expiration date of the lock.
So how to best shop? Well, that's a good question. My best advice is that you've probably got to narrow your choices down to a few lenders, get on good communication terms with each and then trust that they're giving you valid rates that you can act upon when they show you a GFE. If you don't get that vibe from them, find someone else. Once I have a client's loan pre-approved, I encourage him/her to call or e-mail any time for a current and accurate quote, and that can then be compared to any other as well as locked if desired. If the client needs a new GFE it's easy to provide.
I hope this helps, but let me know if not.
Tripoint Mortgage Group, Inc.