A pre approval should only be considered as a guideline for a borrower to better understand their limits with conventional lending. You could get 2 or 3 to see who may be willing to lend you the most... but it is unofficial either way. If you get preapproved by one bank, you are in no way obligated to continue with that institution or loan program.
Going through these pre approvals can be best for giving you feel of the different loan officers available to you. Having a guide through this process that you trust is the most important.
You should not be afraid to shop numerous banks, as all mortgage related credit inquiries within a running 60 day period are counted as one hit on your credit. Don't let anyone scare you away from seeking out the best offer available.