Imagine the frustration of selling your current home and then finding out that something has changed on your credit report without your knowledge that would prevent you from qualifying for a loan to purchase your next home. Maybe it is not your credit that has changed, but loan programs that changed since you purchased your last home. There are many people that were able to qualify for a home loan a few years ago that cannot get a loan today. Stated income loans, bank statement loans and many other loan types that were considered sub prime or alt doc loans no longer exist.
It is easy, and wise to spend some time talking to a mortgage lender and get approved for your new home purchase before selling your current home.