For me, the line is so blurred between mixing/mastering that I don't even think it's possible for me to master my own music unless I physically bounce down a 2track, if I'm doing work on the master channel it's always just part of the mixing process.
I couldn't bring myself to bouncing down a 2track to limit myself so vastly, just to try and get the best of what I can do with the 2track? Not when I have the project at hand.... but anyway
I start at 30 - 40ms and auto release then dial back the attack time to somewhere that sounds good while listening carefully to everything the compressor messes with - transient material first, then punch and then the swing, then mud and low end and congestion, glue, distance, width etc.etc.etc.. I never go faster than 10ms. I generally always leave the release on auto. 4:1 ratio is a fairly safe bet, adjust threshold for 1-2db of GR. I'm starting with the SSL Style compressor. So the vintage VCA in logic.
Me mindful of lowering the ratio to 2:1 with the threshold in place because it changes the whole characteristic of compression because it's directly tied with the compressors knee, the 2:1 ratio applies more gain reduction than 4:1 if you don't compensate with the threshold. I have been using compressors for years without knowing this, so while 4:1ratio sound aggressive, it's can actually be more transparant than 2:1. But either way, listen hard and a/b, maybe bounce a couple different versions. It should be easily apparant what one you prefer if listening back the next day.
Being a rock mix means nothing, have you got a massive kick? or just a clicky kick? is the vocal buried? or sitting on top and up front? Are drums loudest thing in the mix or tucked back? All this stuff matters to how master compression works so you just need to only add it on the master, and do the settings above and see if you like it, if not, leave the compressor on perhaps and mix into gently. If your compressor is swinging too much because of a massive kick, or only triggering when the kick hits, then you can tell the compressor to ignore it by sidechain HPF which is common.
I use mainly for swing, or if I want to have a slightly louder vocal or lead instrument, the compressor pushes it back down a little (along with all the backing stuff) but it does tuck it back a bit leaving a clearer vocal/lead instrument without it feeling like it's too loud but the compressor can't really do any heavy lifting or the whole track will sound weird. (Maybe that's what you want and your arrangement is designed around it though? I don't know).
if you're slapping it on at the end though, when you do anymore than 1-2db's or GR then your whole balance can come apart, reverbs may suddenly become overbearing, the rhythms might start swallowing the vocal
Read the soundonsound article on master buss compression, good advice in there