When you’re mixing in the studio, you can never have enough compression. One compressor is used for a couple of dB of compression on a track, then another compressor adds a couple of dB of compression on a bus, and another compressor glues the mix together with another dB or two of compression on the stereo outs. The end result is a big, full mix, with controlled dynamics, where everything sits perfectly in the soundscape.
In a live setting, however, too much compression can make your mix sound squashed and dull, and it may hinder your performers from playing or singing their best. Keep in mind, in the studio, most of that compression is applied after the tracks are laid down to tape or hard drive — in most cases, there will likely only be slight compression applied to a few instruments and/or vocals during tracking, so that the performers can play with dynamics and capture the right feel.
In your live mix, start out conservatively. Too much compression on a channel will put a “ceiling” on the levels, and your vocalists and players may end up straining to achieve a dynamic performance, possibly even blowing out or damaging voices. And in this day and age of digital mixers with compressors/limiters available on every channel, send, and bus, the temptation to overuse dynamics control can be hard to manage.
Apply compression where it is needed, to rein in specific channels that are overly dynamic. Start with conservative settings, and remember that most performers end up playing and singing louder in the real performance than they do during soundcheck. Don’t automatically slap a compressor across every send, bus, or the master outputs. Listen first, watch the meters, and choose where to apply dynamic control.