You have to decide exactly how fair you think the rate is. If you think you can save .5 to 1% on a 30 year mortgage, then you should look at one or two other lenders. You might want to look at other lender requirements such as escrow, prepayment options, early payment penalties, and if the lender intends to sell the mortgage or hold it. You also need to factor in how long you anticipate being in the residence, because that determines what sort of loan is best for you.
I am a big fan of building banking relationships, and if you intend to be in a community for a period of time, then all that needs to be factored in. We have a 15 year mortgage and the bank holds the escrow account, which lowered our rate. I can make extra payments at any time without having to pay a fee for doing so, and most months tack on an extra $200 to principal. We got a second mortgage through the same lender, and didn’t have any problems.
It’s a real pain to try to straighten out your credit history, and it’s best to not to have to tackle that, unless you have nothing better to do with your time.
Perhaps focus on is getting a good deal on an affordable house. Buying more house than you can realistically afford is what got the housing market into the mess that it’s in. People bought up bigger houses because the rates are good. The best way to save money on a house is to pay more on the principal and shorten the life of the loan.