Another thing which hasn't been mentioned, is the need for consistently good trimming.
A steep turn at a certain angle of bank requires a certain pull on the stick to remain level. How hard to pull is dependent on the angle of bank, airspeed and the aircraft itself. But apart from those factors, there is little else that influences the amount of pull required. So after some practice you should be able to execute a level steep turn consistently without having to refer to instruments at all.
However, if your trim settings are incorrect, then this introduces another unknown variable, and your altitude will be all over the place. In both level flight (but that's normally easier to pick up and correct) and in any kind of turn.