The thing I always recommend to my "clients of lesser practical experience" is this -
Whatever it takes to get your mix the way you like it is fine. EQ, compress, reverb, chorus, whatever it takes.
HOWEVER: Try to stay on the conservative side on almost everything. If you want verb, by all means use it. During mixdown, MUTE IT once in a while. Reverb, unless it's for a stinger effect, is supposed to be a "transparent" thing - Once you're actively conscious of it, you're probably using too much. Same with most effects. People aren't trying to listen to "effects" - They're listening to "a recording" that may have effects on it. Too much effects. verb in particular, is probably the biggest "rookie" giveaway out there.
Same with EQ - If you want a big "tick" on a kick drum, go for it. If you're listening to the recording and every time you hear the kick you go "That's so cool - What a great sounding 'tick' on that kick drum" it may be standing out so far away from the mix as to draw attention away from the rest of music.
Follow me here? What I "normally" do when I'm mixing is make whatever settings I might make, then back off on them a bit.
During mastering, anything that "stands out" will probably stand out MORE after the mastering session.
As far as buss effects, compression, EQ, etc., that's normally a no-no. If you want a bit (-1 or -2dB) of compression on a mix to hold it together a bit, that's not usually a problem. Any more will seriously compromise the mastering engineer's direction. Find individual tracks or groups (drum buss? vocal buss?) that could use it more. Same with EQ. If your mix lacks highs, find out WHERE the highs should be. Overheads? Guitars? Vocals? What actually needs it?
I could go on for several days on this topic, but I've got projects sitting here. For a little more, if you're so inclined, go to...
www.massivemastering.com/assets/pdf/MASSIVE_Guide_to_Mixing.pdf
John Scrip