Yes, knowing when NOT to play is the best thing any percussionist can learn: as a percussionist myself (came up thro' classical orchestral stuff)I know there are times when you have to sit thro 397 bars rest! And some pieces where percussion just doesn't happen.

I once saw a bodhran player who was complaining about his sore wrist at the end of a 3-day festival - I was (then) too polite to tell him the obvious - "If you hadn't felt compelled to play through absolutely everything, you wouldn't be in this state now!" - but I was certainly thinking it!

On the other hand there are some songs even where a good percussionist is an asset and can enhance the song, so I will on occasion invite one or a number of people to join in.

A cajon is on my Christmas list.

And shaky eggs should always be played in TWOS (one in each hand), so you can get counter-rhythms.