Here's how it works on my friends machine: When I'm done with the machine I leave the gantry parked close to the end stops. So when I power it up and enable the servo's, the gantry only has to travel a short distance to the end stops. The gantry makes contact with the end stops on both sides and then each servo backs away the same amount. Since the end stops are the square reference points, and the servos back away the same amount, you end up with servo's that are in sync and a square gantry. The whole thing takes about 10 - 20 seconds. I could go step it up some but why?
My new machine, for all intents, has no flex in the gantry at all. The servo's couldn't force it square if it was out. Hard stop homing there will only serve to sync up the servo's so they don't fight each other.