Pre-approval is required in order to "Lock-In" your mortgage rate. Of course this should not deter you from getting started on the process since an initial approval is NOT a commitment to lend (what I mean is that you do not acquire the new mortgage debt until you sign "closing loan docs.").
So again, if you apply for a mortgage today and receive approval, you're eligible to lock in your rate... In the event that your home goes into escrow on a 30-day escrow, you can lock-in your rate for 45-, 60-days since your new purchase will be contingent upon the sale of your existing home. If your home for whatever reason doesn't sale, you're not committed to a new debt obligation and everything is as they say, "A-Ok!"
I hope that helps!