Don't confuse roundness with balance. I have all four tires perfectly balanced, but at 70mph I get a vibration thru the front. Put it back on the balancer and spun it to check for roundness. Found the flat spot (I know how it happened too). At this point I have the option of shaving the tire or getting a new one. As a friend of mine once said... you can balance an egg, but you can't make it round.
You have to evaluate both. With the quality of tire manufacture now it is rare to have a detectable out of roundness at speeds under 120mph (unless you skid
). If you find you have to put 8oz of weight to balance, you should mark the rim and tire and reposition them. No weight will help a tire or rim that is out of round however.