Bristleworms are great to have in your tank. There's hundreds of species and only 1 or 2 that are dangerous. Those 1 or 2 are rarely found in aquariums. Well, dangerous to corals. They're all off limits to humans. DON'T TOUCH!! They hurt. They do a great job of helping keep detritus cleaned up. As NaH2O said, they're usually a good gauge of overfeeding...or so I've heard. Although I have a 10 QT/Macro tank that I've NEVER fed...not once. It's full of pods, mysid shrimp AND Bristleworms. This tank has been set up almost a year with never a feeding.
If you do find yourself with a population explosion of find ones that are larger than you want in your tank (They can grow HUGE) there are easy ways to manage the population. Some Wrasses eat them. My 6 lined ate them like crazy before it decided to go carpet surfing. You can also take a piece of fine nylon mesh and wrap up a piece of shrimp or something in it....tie a string to it and lower it into your tank over night. In the morning, grab the string and pull it out and into a bowl of some kind. It should be full of nasty lil' bristleworms...and maybe a HUGE one or 2...lol. Keep in mind that for everyone you see, there's probably 50 more...lol