You should balance every tire.The purpose of balancing a tire is to offset the "heavy" spot in the wheel/tire combo once mounted.If not balanced,everytime that "heavy" spot comes around,I believe it scuffs the road a little more,causing cupping that you sometimes see.I am not sure if they are still doing it,but a few years ago,wheel manufactures would drill the valve stem hole where the low spot was in the wheel (hard to do now with mostly alloy wheels)and the paint mark or later,a sticker,was place on a tire where the high spot was.Match the two up and get a lot better ride and balance.