I have found in the past with a root bound pot the plants had a tendency to go yellow. I don't know if this is due to not enough oxygen because of the root density, or because the roots cannot absorb the nutrients efficiently this way. On the other hand, I have seen great grows done in considerably small pots, but never a great grow in an extremely small pot. So, best not to let it go too long, and if you really want your plants healthy don't hold back on a pot change.
TGT