Newbie here. When shooting, I level everything using the NN head. But as I rotate, the head doesn't stay level (the dot moves off the center). Is this a common problem, and I just need to adjust the level as I go or could it be something else? I'm shooting Canon XTI, sigma 8mm.
The bubble level is extremely sensitive - maybe to much so. It's difficult to obtain a true and exacting level so some drift is likely. As long as the drift isn't excessive you should be able to find a happy medium. Using multiple levels is like stack marbles and also becomes very difficult. Only one level is needed - the one NN.
Also make sure your tripod is secure and doesn't have any wobble. A slightly loose tripod head, commonly found with lesser expensive models, will create an uneven rotation.
Let us know how you go.