Mortgage pre-approval and mortgage pre-qualification are not the same thing.
DH and I are slways on the hunt for our next project. We keep a current pre-qualification at all times. It helps to make your offer stronger, because it proves you can get the loan. It does not mean you have to get your loan through that specific lender, nor does it guarantee you the best rates, as they change all the time.
BTW, I have belonged to a great credit union since I was a child. My account number is actually 123-33. I have procured at least 6 mortgages, and they've never had the best rates, so don't take that advice as gospel. I have had good luck with local lenders. I've also done well with a mortgage broker with loans from institutions I'd never even heard of. One thing I do appreciate is getting a loan from someone who intends to keep it. My first mortgage was the smallest I've ever had. It was sold repeatedly, which was a complete pain in the ass.