In an ideal situation you would step up little by little in pot size, however I've actually talked to someone with a masters in horticultural studies if that means anything, and his opinion was that it comes down to personal preference. I go from aero/rockwool to 1 gallon to 15 gallon. He goes from 1 to 5 to 15. When I do make the transition to 15, I would say the plants are most definitely at least somewhat root bound. Even transplanting skinny guys about a foot tall, there are roots all over the edges of the pot. I can still get significant growth beyond this point, but not sure if I would get more if I used an intermediate pot size.
Another side note is he really likes the grow bags. He says that it causes better roots. While I assume oxygen to the roots is what most people think is one of the better benefits, he says that its also good because when the roots reach the edge of the pot they actually die at the tips, and it causes them to branch similar to the way a plant branches when topping. This is opposed to the way the roots will just continue to wrap around inside the edges of a traditional pot.
