Originally Posted by Brian Ratliff

Your weight balance does not change significantly until we start talking about massive changes in position; on the order of several inches. A TT position (non-UCI legal, which is typical in the amateur ranks) shifts the center of gravity of the rider forward by over two inches. That kind of shift can affect bike handling, but it certainly is rideable. Smaller shifts of less than an inch can't really change very much. A shift of 10mm in saddle is only a change of 1% in weight distribution between wheels.

As far as standing and stuff on your bike, if little changes affect your ability to move around on the bike, you have issues besides mere bike fit.