Your legs should be at LEAST shoulder width. But yes, the closer the legs, the more emphasis on the quads; while the wider the stance the more emphasis on adductors and hammies.
And I'm gonna go against the grain here and say that you don't have to go to 90 degrees to squat (although I do). It simply depends on what you're looking to get out of the exercise. If you want heavy quad stimulation you should go with a more narrow, higher squat (ie, 2/5 squat so to speak). It's shown time and time again, that the quads are the most active in the beginning of the movement with the glutes and hams coming heavy more at the bottom.
If you want the nice hormonal response, I recommend parallel.
Why not deeper?
Stress on knees. Unless I personally know a person, and can watch/train them I wouldn't suggest something that can VERY easily be done wrong, even when done right.
I agree with unproper form, but in my opinion, the squat is made out to be some ridiculously hard exercise. Does it take a truckload of effort? Hell yea. Is it next to impossible to execute? In my opinion no. I don't stop any of my other exercises at half range of motion, why would I do it for squats?
In my opinion, which you can take it for what its worth, cutting the range of motion of the knees short actually can be harder on the knees. You are also missing out on glute activation if you cut the exercise short. Full flexion/extension is the natural range of motion of our knee joint that we were born to use.
I also wanted to add to the OP, I recommend gripping the bar rather close. Most like to go wide due to lack of flexibility in their shoulders. By doing this, your elbows are going to point backwards which will automatically create excess forward lean. Grip closer and have your elbows pointing straight toward the ground.
I agree with the ROM, it's muscle specific. Whatever you want to get out of the squat, determines your ROM. As for the stress on knees at a shorter ROM, I've read literature to back this so I can see your point; but I personally agree with the opposite.
I go wide grip, and had to correct the forward lean myself when I first started, so that's definitely a good piece of advice if you fail to correct your form yourself.
I also wanted to add to the OP, I recommend gripping the bar rather close. Most like to go wide due to lack of flexibility in their shoulders. By doing this, your elbows are going to point backwards which will automatically create excess forward lean. Grip closer and have your elbows pointing straight toward the ground.