To be fair, i've used 390rpm before and not had a problem! The only thing i would say different to Jason is, if you have never used a rotary on carpet before, then i would get a slow speed first! 175rpm-200rpm, before you go to a speed of upto 400rpm, as there is potentialto cause damage if you have experience!! i'm sure you can do damage with a slow speed, but your more likely with a high speed!
Just my opinion!