In PA it's the law but very few i know register ther quads! I DID IT SO IF IT GET STOLEN, AND FOUND I CAN GET IT BACK! However it is also needed to ride on most of the atv trails that the DCNR designates (which i have yet to try)!
Just went there this weekend! Funnest place I've ever rode! Gotta have a whip and flag and $5 a day for ORV permit. That's all! No registration required other than the permit! 4000 acres of open sand, trails, camping, fishing and LOTS OF DRAG BANSHEES! Good Times!!!