I didn't read all the replies, but I use to work in sewing factories. We always cleaned our machines at the end of the day and put fabric under the pressure foot. The fabric under the foot was to catch any oil drip so at the start of the next day when starting to sew, there would be no oil on the product.
For my home machines I don't put fabric under the foot, but I do put the foot down. I don't believe it is good for the spring to leave it up.