Hey there,
I’m very new to ZBrush but I really like it. I’ve been watching and reading tutorials recently. So I tried modeling a head from a “squircle” (a 6 polygon square devided once) and it ended up looking very cartoonish, but I had trouble modeling the mouth and eyes because of bad topology (how I understand it):
What is the usual workflow for modeling a head? At what point do you retopologize?
Thanks 
if you feel that your sculpt is getting constrained. and if your overall large form is now finished.
Work at lower subdivisions before you get so far in the detail. Get the main shapes right. After, define the shapes of the bones, the muscles and the skin.
If you don’t have a tablet, think about buying one if you want to get further in ZBrush, well, in my opinion.
There’s a good issue that has been recently release, it’s ZBrush Essentials. It’s 35$ but I learn really good tricks in this, the first tutorial explain the basic workflow for a bust.
There’s plenty of info on pixologic and ZBrush Central anyway. like here
http://www.pixologic.com/docs/index.php/Sculpting_a_Head
have fun! it’s an important part of the process!
www.nicolasdoyon.com
Thanks guys for your answers, I do have an Wacom Tablet (graphire4) and I did make the basic shapes at lower subdivisions 
And btw… at one point when I had saved the tool and imported it back in when I wanted to work on it again, the symmetry was thrown off center, causing the crevise/groove between the mouth and nose to get a bit weird at the bottom when continuing to sculpt. Can you think of a reason for that?