ORIGINAL: Campy
Why do a lot of people try to complicate things ?
Balance your plane with the gear retracted, which is the way you will normally be flying. When the gear is down, MOST retracts will either move the wheels forward slightly or they will stay in the same position relative to the CG. If the wheel moves forward (MOST retracts ), this will make the plane slightly nose heavy. While this will make the plane a little sluggish, it also makes the plane more stable, which IMHO is good for takeoffs and landings
It would seem to me that the CG wouldn't change at all. The retracts are using the same hinge location no matter if they are up or down. It shouldn't matter to the CG.