I don't know how you could finish the plane w/o balancing it. How do you decide where to put the servos? Repeat after me, "Lead is evil, weight is bad..." Say that five times. With some planning you can almost always avoid needing to add weight. Once you have everything built, but before the radio is installed and the model covered, assemble everything including the tank, engine, prop, spinner, muffler and so on. Then select a servo placement that will put the cg as close as possible. Then place the receiver and battery to bring the cg the rest of the way in, or if it was already balanced, to keep it there. Here's the key, though. Don't permanently affix the receiver and battery if you can help it. That way you can move them around to make any necessary adjustments once the model is covered. Unless you are covering the model with sheet metal the covering shouldn't change the cg very much.