Sunning them wet definitely works. (I don't know if dry works or not, sorry!)
You CAN sun your PUL covers, but I don't think there's any reason to unless you're already hanging your laundry out to dry. Sunning them rather than machine drying them should make them last longer but I don't think there's any real need. For what it's worth, I have found that sunning my Thirsties covers had NOT gotten rid of the poo stains on the white parts. :P (Works like a charm with poo spots on prefolds, though.)