The shins should be as vertical as possible in any squat. I know not everyone can do that, but you shouldn't TRY to jut your knee out. To the contrary, the lower you go, irrespective of knee position, the more muscle you recruit, because the lower you go the more hamstring is recruited. You will get the same quad stress (minus the knee stress) by keeping the knees over the toes as you will way in front of them. Plus, you get to keep your knee caps until you're old.
For me the only difference between overhead and zercher is the amount of ab strength required to balance.