It is very important to make sure your mixdown is properly done before it is mastered. Why? Because your sound will not be determined during the mastering process. You, the producer, determine the sound of your track and you should have a clear idea on how to achieve that in your mix. How much reverb do you want on your vocals? How compressed should the snares or the kick be? Do you want to have a dreamy sound or should it be dry? These are the decisions you need to make during the mixing process.