We locked a few weeks ago at 6.875 when we were 30 days out. Here is how we made our decision:
1. We intially budgeted based on the rates going as high as 7%. Since we were able to lock at less than 7% we did not want to risk it since some banks such as B of A were already quoting 7% rates
2. Our loan officer who has more than 20 years experience did not believe rates would go down to the original 6.25% quote we were initially working with when we started building in Feb
3. We can always refinance with no prepayment penalty at a later date if rates go down.
I forget what the exact amount is, but there is a certain dollar amount in interest you save per 1/8 of a percentage point. To us, since we would probably not see 6.25% any time soon, assuming that MAYBE the rates went down to 6.5%, the money saved per month was not worth the risk to us. Also, if your credit scores are good, but not quite 720, you will not get the best rate available.
This information is based on a $300k home with 5% down.