If the question is specific to nxt only, tag it nxt. If it's more general, but your use-case is the NXT, then tag it with both. It's more general, tag it with mindstorms only.
For example, my question from yesterday, How much depth does the NXT battery pack add? I tagged nxt, but NOT mindstorms. The mindstorms was added by someone else, and I would argue isn't needed for that question, as it is very specifically about the NXT and not applicable to any other mindstorms series.
Then How to get started with Mindstorms? is mindstorms and not nxt, which, due it its more general nature seems right to me.
With Can I use RCX sensors and motors with the NXT brick?, I think it make sense to have mindstorms+nxt+rcx as you're crossing mindstorms lines and beginning to get a broader scope.