I have no problem with putting compression on a mix if it needs it. A little bit (1-3dB) of compression in a mix can be just what the doctor ordered to glue it all together.
HOWEVER - Yes, draw the line at some point around what the mix needs - Don't compress for the sheer sake of volume. Leave that for mastering, whether you're doing it yourself or your sending it out.
If you need to hear it *louder* during mixdown, turn the monitors up.