Well, if it's to far back, the plane becomes tail heavy and you will never have a successful landing, or at least it will be one heck of a hand-ful trying to land.. and fly too.

To nose heavy, well, if at altitude, and you cut the throttle, gravity takes over and landings can be a tad bit fast if not difficult.. if the weight is to much forward.

Stick with the numbers given by the Mfg, varying those only a very small amount, say an eighth to a quarter of an inch at most, at a time to get it the way you want it. Slightly tail or nose heavy is not a problem.. but to far back or forward can really be a handful to fly and could actually make it a tad dangerous.

DS.