I will usually try to put all the things that need to be controlled and major components like fuel tank and engine in or on the plane before laying out the radio equipment. it's good to have all the "endpoints" of the control system in place before deciding where servos and electronics can or will fit. locations of these items can and will greatly affect balance and I always try to "build to balance" rather than just adding weight regardless of where the plans show things should be. you usually can avoid allot of unnecessary weight by putting stuff where they do two jobs. I also try not to get too anal about exact balance point during building. I don't I've ever seen a plane make a maiden without something being moved around afterwards. even if just a little trimming is done to get the plane to fly right, things can usually be moved a bit to eliminate (or at least reduce) the need of trimmed control surfaces. you simply cannot nail a c/g down to "1/32 of an inch" because the balance point changes constantly as the plane flies and everyone builds slightly different so no two same planes built by different people will have exactly the same balance point.