Are you volunteering yours?
Everyone knows the answer to that, too.

Really, what kind of vixen (or fox of any gender, for that matter) would I be if I
didn't volunteer my arse?

Frankly, I dislike having my arse licked. Feels too passive to me.
Each to their own. Personally, I rather enjoy being able to just lie back and let someone else do all the work. (Though admittedly a mere rimjob is very rarely "all the work" I want done.)