from the sound of things i think you mean will you need an alignment if you change from 19's to 20's? unless you already need a alignment i wouldn't think it is needed.
But if you really are talking about rebalancing rims and tires, they need to be rebalanced everytime you put a new/old tire on each rim. When buying new rim and tire packages, it's usually automatic that they rebalance them.
What size tire are you trying to go with? On the 19's, a 245/45 is perfect so on a 20", a 245/40 is perfect, based on a 8" or 8.5" wide rim. The diameter of a 45 series on a 20" rim ( unless on 225mm wide) may be to large definitely when lowered, more rubbing problem will occur, I already am with in 1/18" when turning, I have a lot of rub marks, I just don't want to see you regret your decision.