I've found, from my past experience with the registry, that unless you're deleting something that directs a crucial system file, deleting something--such as, in this instasnce, Napster's registry folder--you're safe.
Those files that don't immediately manage the core of your system are usually just left after the uninstallation for basic tweaks or settings. This makes a program like Napster rendered with all of your personal settings--like the installation path and such--once it's reinstalled. And, usually, with registry files like those, if deleted, upon installation, they will be written all over again. Meaning, in short, you're most likely at a loss to delete them.
As everyone before me has logically stated: <b>do back up the registry before you take any course of action</b>. Go ahead. Export the Napster registry files. Furthermore, I propose you find a secure way of backing up anything you need to keep on your system. Then delete the Napster registy files. Restart. If nothing else, you'll find out, first hand, just how Windows works without them. ...And, no matter the outcome, if you've backed up, you wouldn't have lost as thing- only left to gain a bit of experience using your registry.