I was going to post this exact same question about a week ago, then reread the SF FAQ page (as referenced in the other answers).  The distinction I use is how potentially wide-spread I see the problem in the question.

From a desktop support perspective there are certainly cases where a large(r) number of machines will be affected by something, in which case it's appropriate for SF.  There are then others that are isolated to a single computer, these tend to be the ones I vote to move to SU.

That won't always be the case, though.  For example, you can get one PC that's not getting a domain group policy applied correctly - more appropriate for SF.  In these cases I'll usually leave a comment on the question stating why I think it's appropriate to leave it on SF.