You'll find that a shop mechanic will tell you to do both chain and sprockets. Basically the sprockets are right there while doing the chain. So it pretty much gets covered by the same labor charge. Sort of like doing the timing belt while replacing the timing cover gasket on a car. If you do your own work, it doesn't much matter. Other than that, the factory service manual will say replace sprockets when they are worn. A small amount of wear might happen if existing sprockets are reused. I've never really seen anything terrible happen, unless the rear sprocket is not lined up right, and then the sprocket and chain wear out terribly fast. I'd say a dirty chain and sprockets will wear it all out faster than anything else. Sand and grit and gunk will do it. Riding through rain will get all sorts of grit up off the road and all over the chain. Wiping it and lubing it after riding in the rain is a good idea. Do not ever run the engine to turn the rear wheel to wipe or lube a chain. The chances of losing fingers is extremely high. Like I said, I've worn out a few chains, but never a sprocket. And haven't seen a chain wear much faster for using the old sprockets over again. I've never had to buy more than one chain for each bike. (Except that HP modified bike that ate chains.) But even I'm considering changing the sprockets now, just for fun. I've got a new chain I've not put on yet. I'm now looking for rear sprockets... Maybe something UberGeeky... And yeah, http://www.indysuperbike.com/customer/home.php was the cheapest place I found for chain and sprocket sets. And they offer the DID and a couple of other excellent chains. Ok, the 520 chain and sprocket sets are hard to find on the Indy website. Look here: http://www.indysuperbike.com/customer/product.php?productid=1000000&cat=34118&page=1