I have a long hair full cap system, and conventional wisdom has always said, "NEVER comb it when wet!". My real world experience however has been that I lose far less hair detangling it while it is still wet than if I waited for it to dry. You lose hair either way, but as JRob says, if the comb slides through the hair while it's still wet, it's a good thing.

And I've never found that using conditioner first before combing/detangling causes more hair to come out or get unknotted. It makes the detangling much easier. Just take it slow and gentle and everything will be OK.