I'm a big fan of Mixed in Key but I don't really worry about the keys of songs on the majority of my gigs. If you are doing a Mixed house or trance set with long blended transitions between tracks it is really useful to avoid horrendous sounding key clashes, but on most of my gigs I just do short cuts between tracks so harmonic mixing isn't all that necessary.

I find the energy ratings handy on all my gigs though. All my songs are tagged by key and energy rating and it helps me minimise the risk of killing a busy dance floor by dropping a song with a big drop in energy or playing something too energetic when I'm supposed to be playing low-key background music.