There's no substitute for getting your mix as good as you can before sending it off for mastering.  It's never a good idea to subscribe to the theory, "that's good enough... they'll fix it during mastering."  Your mix isn't ready to be mastered until you're completely satisfied with it.  If there's a problem with a track, redo it.  If there's a problem with the mix, remix it.  If you leave the problem for the mastering engineer, the results may be less than ideal because mastering is a series of compromises.

Mastering is the process that insures your mix will sound it's best on the widest variety of playback systems.  On a mix that comes from a professional studio, the mastering engineer will probably make EQ adjustments in .5dB increments.  In a worst case scenario, it may be as much as 2dB, but that's usually the maximum.  A mix that comes from a home studio is usually a different story.  On those, it's not unusual for a mastering engineer to perform corrective surgery.  What's the difference between minor adjustments and corrective surgery?  It has to do with monitoring and room acoustics.  Professional recording studios (and mastering facilities) spend thousands of dollars to get the room acoustics just right before gear is even considered.  In home studios, gear is the main focus while acoustics are less of a consideration.

A mix engineer makes decisions based on what he/she is hearing.  In the pro studio, adjustments are made based on a good combination of monitors and room acoustics.  In the home studio, even one with good monitors, poor room acoustics can cause the mix engineer to EQ frequencies incorrectly, especially the low end.

If you're using a home studio where the acoustics are less than ideal, don't despair.  Strive for a good sound based on what you're able to hear.  What you want to avoid is leaving something you're dissatisfied with for the mastering engineer to try and fix (vocals to loud, to soft, etc.).  The final result will always be better if you handle those situations while the mix is still in your hands.  For the problems you don't hear due to monitor/acoustic deficiencies, mastering will come to the rescue by performing corrective EQ adjustments.