IMHO, any raise in the needs needs to be matched at the front otherwise you are changing the rake and trail of the bike. Raising the rear without raising the front will typically make the steering "quicker" but slower to return to a neutral line. head shake can occur if rake and trail change is sizable.
side note, if a longer shock is added and then sag is set so the bike returns to original rake/trail values, the result is typically that you are back to the original shock travel.