In my case I started without locking mine. 383, 11.3 compression on alcohol. When I did lock it out I found that it started easier and would build RPM faster, I.E. better throttle response. This is on a drag only motor. I didn't gain anything in e.t. because leaving either off of the foot brake or trans brake, in my case, I was already above the RPM to bring the advance all the way in. Coming up against the brake was better/quicker though.
Curtis
P.S. I changed nothing else but locking the distributor so I would know how it acted differently.