You won't need a class rating specifically for a PA28 except in the unlikely event that your SEP Rating has lapsed. IIRC a sign-off via a check-ride from an instructor is not legally required but is a good idea if you're unfamiliar with the PA28 or low-wing aircraft in general. A check-ride and sign-off would be required by most aircraft owners considering entrusting their baby into your hands! But let's face it, you could go and buy a PA28 of your own and nobody but your conscience would force you to do a check ride!
