ATC does not see VFR flight plans. All they see is what the initial controller types in when you request flight following. Also, if you change facilities while receiving VFR flight following (i.e., handed off from Philadelphia to Baltimore) rather than just changing sectors within the same facility, the new controller may not have the flight data from the previous facility. In that case, they'll ask you for your destination/route of flight and maybe even give you a new transponder code.
That said, due to post-9/11 security concerns, any change of destination must be questioned, and if the pilot's response is suspicious, reported to the Defense Emergency Network for further evaluation. So if you say you're going to Point A, and then turn in another direction without informing the controller, the controller may ask what's going on. Best bet if you leave the route of which you'd previously advised them is to let them know either before or immediately after you turn. And don't worry about your explanation for changing destination -- it's OK to say, "I gotta pee."
