I fine-tune my stance by doing 2-3 reps without a bar and adjust my feet as narrow as I can while still going down all the way. Once I get that "feel," I start squatting. I end up with my feet being ever so slightly more than shoulder width and toes pointed out very slightly.
Incidentally, I have heard from numerous sources that putting plates under your heels is a bad idea. It has to do with adding more strain to your knees or something along those lines. Frankly, I do not like the idea of something that can move under my feet, so the idea never appealed to me.