Agree 100% with JasonTX about bending forward before the snap. It gives the wrong impression and my opinion is that it is not a good way to be ready to move if needed.
I think it can certainly help before the snap with Umpires. Leaning forward a little can help to stop Umpires being skittish and dancing around on running plays. Pivoting on the spot to watch a run go past you is fine, but moving about too much on a run may lead to you getting accidentally hit by players who are trying to get past you to make a tackle.
Personally I try not to do it at the end of a play as I was picked up on this by a Div 1 official at a clinic scrimmage. His attitude was to be stood upright and always ready to move - "you don't know what will happen next so be ready to react to whatever might happen".
But at the end of the day whatever works for you and helps you cover a play to the best of your ability is fine, as long as you are ready for the unexpected
