Obtaining pre-qualification for a home loan cannot be stressed too much. Not only does it assure you that you are looking in the correct price range, it also assures the seller that you are a bona fide buyer. You can shop around via the telephone for a loan representative that offers the service and loan programs that suit your needs, and even be pre-qualified over the telephone.
For full pre-approval, however, it is important to select one lender, fill out an application, and possibly write a check for the credit report. Searching for a home can definitely start in advance of the loan pre-approval, but it is best to be pre-approved before submitting an offer to a seller.