High or low speed for Dremel cut-off wheels

When using a cut-off wheel for a Dremel or similar tool, should I be running the Dremel at high speed (high RPMs) or low?

I don't know that the application matters, but I'm cutting a bunch of bolts. I want a decent job, but I'm not worried about aesthetics. I guess I'm mainly interested in the long-term health of the cut-off wheels (though, of course, long-term for an abrasive wheel is pretty short).