If you are flying VFR and controlled airspace is not an issue and you are out of the circuit then it is not necessary to keep a constant altitude. If you have trimmed correctly and you gain or loose height due to turbulence don't worry about it - the aircraft will settle itself again after a while, as long as you are not constantly chasing the airspeed or the altitude.
On the other hand, if you are in a situation where you do need to keep a constant height, then adjust your height immediately. Don't wait. Be careful not to over-correct.