It wouldn't be in your thread tools if it weren't allowed. We don't have too many people who sticky their threads, so it's never really come up before. FOR NOW the official word is that it's okay.
If you ever post something in a forum that has sticky threads, though, make sure you don't mess up the order when you sticky yours. You can set the sticky order when you sticky it. (And if it's something like the Roleplay Institute or Worldbuilder's, it's probably good to ask a Staffer first!)