If you are going to rotate your tires, do you need to balance the tires again? I was thinking that once the tires are balanced, they didn't needed to be balanced again a second time even when you rotated the tires.
When they balance tires it's done on a machine not your car so if position mattered how would they balance tires? As long as the balance is done on a high speed balance machine and none of the weights come off or the tire is out of round you should be OK.
It's not that the position makes a tire out of balance or not, in that respect you are correct. It's that if a tire is out of balance, what you feel (or not feel) is greatly affected by the tires position on the vehicle. Generally on the back, you'll feel the vibration in the seat and when a front tire is out of balance it transmits through the steering and you feel it in the wheel. Many times a tire that you would feel on the front of the car, you may not on the back. Also many times an out of balance tire will create a harmonic so to speak and only be noticeable within a certain speed range. I once had a client with a 944 tell me he got a tire vibration, but only between 90 and 110.