My car occasionally pulls to one or the other side as well but it's just the ruts in the road and the low profile/stiff sidewall tires that cause it in my case. On flat pavement it stops just find with no steering input. I can let go of the wheel and it's completely fine stopping in a straight line.
I'm with the two suggestions above, go find a flat smooth parking lot or something similar (preferably not a road as most all roads have camber built in so they slope from the center down to the sides to drain water) and try braking from different speeds there, see if it pulls. Try the same stretch of pavement in both directions to see if it pulls the same way each time to make sure the road isn't causing it. If it still pulls the same way with all your tests, it's probably time to take it in for an alignment.