Two things- first, no legitimate lender will give you a pre-approval without knowing what your credit scores look like. I'm not implying that you are assuming anything else, just want to make sure you know. Second, take a look at the link below from the FICO website, it should clear up some of your concerns. You will not see a decrease of more than a few points for shopping a mortgage. Scare tactics of major credit score drops from rate shopping are garbage.
I also have a slight issue with what Mike said below. Lenders do not "feed lines" about guideline changes. The truth is that they do change, and I would not put my name or my bank's name on a pre-approval or any other paperwork that was completely useless because it was based on a hunch. Pre-approvals require documentation.
E Mortgage Management
800.793.9633 ext. 156