If the mix has good dynamics to begin with (example) Peaks of -1dB and RMS of -18 or so..Mastering can squeeze it up some..if tasteful. All too often, mixing engineers are adding 2-bus compression to their mixes to get the client to a satisfaction level in overall volume. When I get one of these, I have to pull every dirty trick out of many hats in order to restore dynamics to a level that is masterable. Equilazation can restore the dynamics..but you have to be very careful not to destroy the mix. Sometimes the mix is destroyed before I got it

but really, see if this makes sense.
If the project is to be professionally mastered, give the clients a one off with some 2-bus compression (for clients sake) of the mix and send the non-compressed OM to the mastering house..or if you are mastering it, work with the uncompressed OM.
Now not to confuse things, channel compression is fine (and necessary) on certain instruments. This is not to be confused with 2-bus compression.
Mastering engineers can do a far better job of your work if the 2-bus is not squashed. Leave some dynamic range. I actually request a max level of around -3 peak on mixes..if I can get them. I am hearing some improvements along these lines. I am hoping that more mixing engineers will look at the above formula and take it into consideration. Really, I have seen CD players that clip at the line out at 0dB.
What happened to the glorious days of -17 BEING 0dB RMS reference? Well that was the intention when digital was being utilized in 1976. Ry Cooder "Bop till you Drop" was pressed in 1976 and it was a Digital recording. Around 85, newbees to digital recording (some of the old school tape pushers ((not you Bob!)) decided to slam the brick wall...then it simply got out of hand and the CD manufactures (consumer machines) had to accomidate this higher level of mastering and then the digital domain as we know it does not adhere to the -17 "ideal" as intended..and it may never go back.
If you have some CD's that were pressed in 83 and 84, you will see what I mean.
Bob, care to chime in here some about this?