Thats just the thing, we discussed this once before... Personally, I like mixing but if an artist comes to me wanting a finished product, I strongly suggest that he allows me to do the final mix before mastering it. Saves me a lot of time on trying to fix ish in the mastering process. I prefer to fix it in the mix if I have that option. A lot of engineers like to separate the processes (which they are 2 different processes) but I hate going behind someone elses mix (if we're not on the same page) and try to compress ish in the mastering process. I'd rather do both.
As for compressing in the mix, I will do it if it's the only way to get the correct results but if I can avoid it, I do.