Dave is correct (as usual
). I think what often gets overlooked is the fact that the inside of the pulley rides on a bunch of rubber. There is some play at the rear pulley. The rear wheel rides on 2 bearings and a solid axle. There shouldn't be any lateral movement. That's why I'm not 100% sold on trying to center the belt at the expense of wheel alignment.
I'm not saying that the lines are perfect or that you can't fudge things a little bit. But to take that to any extreme would be foolish in my opinion. If you have to do anything drastic to get your belt to run quiet, you have bigger issues and you might want to look at solutions like the ones Batman and Dave mentioned.
Has anyone every run a laser down the outside of the rear pulley to the outside of the front pulley?