i spent some time worrying about balancing my wheels, but i blew it off. i used to work at a hi-end tire store years ago, and also raced SCCA autocross and solo racing. we had tire matching grinders, on-car balancers, all the goods.
one of the things i learned, is that the frequency, modulation, harmonics, whatever you want to call it, occur at about the same speeds, usually at 33mph (+ or - a few,) then again right near 60mph (and again at greater speeds, but we're not concerned with that here.)
at those two speeds is where you'll feel your car start to shimmy if your wheels are out of balance (this isn't taking into account bent wheels, warped rotors, loose parts, etc.) when you power out of these ranges, your vibration goes away.
given that these bikes top out when the vibrations would occur, i don't think balancing is justifiable, and just another thing to get all excited about for nothing. they don't balance the wheels on racing bicycles, either.
if you could somehow dynamically balance a bicycle wheel, one good skid, a pothole, anyhting that could slightly deform your wheel would throw it out of whack.
what 2door's talking about is "static" balancing, which is basically finding the heavy spot (usually the valve stem) and compensating for it on the other side. in theory, this would work on a bike wheel, but i don't think you'd feel any effects except psychological.
a properly trued wheel with correct spoke tension and smooth tires are all you really need.
if you went thru all the trouble to balance your wheels, and then you're rockin' some dirt grabber knobby tires, you just wasted your time.
even if i could get my bike to 90 mph, i wouldn't worry about balancing the tires, because since they have no speed ratings, they'd probably blow off the rims anyway...