I think the Starting Strength book mentioned plastic plates and that their purpose was to position the bar off the ground properly so that you can learn the movement with the correct form even before you can lift the heavier weight.
The Starting Strength book also said that if the bar hurts your back on the squats you aren't tightening the back muscles enough before you put the bar on your back.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Current squat: 162.5 x 2