For the common programs, the differences in rates between reliable Lenders is minuscule. Each of those 24,000 lenders have the same cost of funds' on any given day. You want a lender who will sell' you FNMA money at the lowest mark-up. Read them and ask questions until you understand them. While this disclosure is not binding on the Lender, it will show the costs and fees on the program on the date you applied, a good starting point.
Many people think that their rate is locked in at the rate shown.
You need to find out your lender's lock policy. The market can change rapidly so developing an executing a good lock-in strategy is very important. If you are dealing with a Mortgage Broker, insist on seeing the rate sheets from his sources of funds. If he refuses to show them to you, get up and leave. Then get an Agreement in writing that they will not charge you more than the agreed upon commission.