I've never has a problem. Over the years I've used rags, paper towels, steel
wool, scotch brites, and pretty much anything that will carry the finish to
where I want it.

I usually try to be sure that whatever I used has decent air circulation after
use - until it has polymerized anyway. But sometimes it just ends up wadded up
in the trash.

maybe I've just been lucky, and I certainly wouldn't advise being careless, but
in my experience, the risk of spontaneous combustion is pretty low, so I guess
its a matter of your personal risk aversion....

-J