I make mixing part of the creative process when I am producing a song. I generally do some basic EQing on a new sound right away (e.g., if I add a hi-hat, I will automatically cut the low frequencies that could potentially create phasing problems with the low-frequency dominant sounds already present in my mix). Once the basic EQing/filtering is out of the way, I can get more creative to shape sounds in interesting ways. Doing all of this as you write and arrange a song is advantageous because nothing is finalized yet, and there is room for experimentation. This can yield remarkable results you would have otherwise not achieved.