Actually, picking pin-by-pin will also leave traces. The act of levering a pick against a ward, or the bottom of the keyway, causes the pick tip to move in a small arc. This movement creates small striations on the pins as they are raised, since the pick material is harder than the brass pins themselves. Polishing picks can help with the longevity of your practice locks.
Try this:
Get a brand new practice lock and pick it, pin-by-pin, once. Strip the key pins out, then examine them with a magnifying glass in good lighting...
Or:
Get an old lock which has not been picked, which has a good oxidized coating on the pins, pick it once, then strip out the key pins. Examine them with a naked eye...
You'll see.
