Hi
George, there are two schools of thought on this issue. Some say it is OK to apply mastering effects while mixing and others say add no processing to the master buss while mixing and apply the mastering effects later in a separate session which you call the mastering session. And wait some time too before doing it.
The advantages with this is that if you apply no mastering type processing for say a week you get to hear your mix etc and you will often end up wanting to change things later and perhaps remix stuff. I can guarantee you will want to alter things after listening for a week or so.
Also it is well known fact that after mixing for say 8 hours or so is rather fatiguing to the ears and this is not the best time to be applying any sort of mastering processes. You are just not in a position to make decisions regarding important things such as overall EQ for example or even compression. When you leave it for a week and then get into applying the final mastering processes you are in a much better place to make judgements about overall EQ and compression etc. It is like you are hearing the music fresh and for the first time and that is something that is very valuable to me for example.
But in saying all that as well I am quite partital to applying some very slight compression to the overall mix but only in a very subtle sense. I am talking slow attacks, low ratios and low amounts of gain reduction. This can help in certain situations.
Think about this. Usually in the most professional of situations a separate mastering engineer is usually involved and they will usually ask for the mix with NO processing what so over being applied. Doesn't that tell you something? And if you are not into using a separate mastering engineer that is fine too, the next best thing you can do is apply no mastering processes during the mix, wait a period of time and then master it yourself. It will still work out better than attempting to master stuff while you are mixing. The problem with the guys who do all the mastering while they mix is that they have never tried it the other way. If they did they would change their minds big time. They are in too much of a hurry.