For my pop/variety group, no compression, the same goes with big band, jazz combo, and most styles! For my metal group I use my Empress and Darkglass VMTD ,both always on as it helps me sit in the mix *so* much better with twin guitars!!
All that said, I don't buy the whole idea that people who use compression "hide" behind them as a crutch for poor technique which is similarly used as an reason *not* to use overdrive!! I'd argue that either *can* actually bring technique problems to the surface.
I've gone back and forth on this and found I'm completely fine *without* in any scenario, ive also owned my share of bad-terrible comps...So is compression necessary for live use?? No, FOH will almost certainly compress you, but it sure makes a difference once you find a good one, for me the Empress is just perfect!