you can go from start to finish in those cups if you want. There is no "should" transplant, just "can".er root systems, but it's not necessary if your situation dictates so.
I'm not sure what i think on this one. I frequently took a youngun grown in soil, to move to hydro, and upon very carefully washing off the roots in the bath, the above ground growth always hugely surpassed what i found in the soil. Who knows. It would often be a 6 inch plant with a 2 inch root ball. Yet at the same time, i've had a 4 inch plant in a pot twice the size of a solo and found roots at the drainage holes. so yeah, i really don't know where i stand on that statment. Am i allowed to argue both sides?
the easiest way to determine when your plants need to be moved into bigger pots is to tip the cup over and let the plant slide out and into your hands. look at the roots. is the pot root bound? are the roots circling around the bottom? If so its time to move them into a bigger pot. don't be afraid of pruning some of the root mass. or just loosing them up a little.