Rootbound is a tricky thing to understand.
Sometimes its ok to say thats all the root space you ladies get, and sometime the Plant Will show that it needs more...
In alot of ways you can grow 1 or 2 foot tall plants out of 16 oz dixie cups, but this will effect final yeild versus a plant that with same light air, nute that has 5 gallons to reach into.
In High Density sog you really dont pot up. However in lower level sogs you can.
It really is personal choice, but just try to pick a appropiate pot size for your ladies to enjoy for the end of thier lives.