Here are some excerpts from a few places on the net:
MYTH #3: I can isolate my quads better by using a block or weight under my heels.
SHATTER THE MYTH
The practice of putting blocks or weights under your heels is widespread among bodybuilders in order to gain better isolation of the quads while squatting. The problem is that your knees go way out over your feet,
placing great shear and compression on both the cartilage and ligaments of the knee, which over time can cause serious damage to the knee joint.
Above taken from:
http://www.topendsports.com/fitness/technique-squat.htm
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Downward Phase
Slowly lower the body by flexing at the knees and hips. The torso is maintained in an erect position. Weight should be distributed over the middle of the foot, NOT on the balls of the feet or the heels. The heels should remain in contact with the floor at all times. Continue to lower hips until the tops of the thighs are parallel with the floor.
Keep in mind that the knees should not protrude over the tips of the toes. Do not bounce at the bottom of the lift.
Above taken from:
http://www.athleticadvisor.com/Weight_Room/squats.htm