I've used scuff pads (basically Scotchbrite) to rough up plastic parts (for example, the underside of the front fender) and for hard parts like brackets. I don't recall using them on the top surface of a sheetmetal part. I've always wet sanded those with 400. In 2002 when I had my original tins stripped, my buddy spray on a couple coats of primer-filler and I wet sanded them like that, then he did the sealer, paint and clear. Yes, I'm just a sanding monkey.
Generally speaking I'd go with what the paint supply dude says. At the very least then you have someone to throttle if it goes wrong.
