Cutting your 20 mg is just fine...but, if you do feel better to have the accurate 10 mg (or 5 mg so you can take 2 of those), just ask! You are never annoying if you ask your doctor for these kind of simple changes.
I mean, it's so simple for your doc, I'm sure he/she won't mind: and you might feel much better (mentally) if you take accurate dosages.

However, please keep in mind that prednisone needs a close watch; so if you have pills with 20, 10 and maybe even 10 mg, it can be easier to mess up (if you are so familiar with taking 3 pills, there is a change you take 3 times 10 mg or something like that).

Anyway, I always use the 5 mg, even if I'm on high dosages, because I take them early in the morning, and well...it's a big difference if you take 8 pills with 5 mg each, or 8 pills with 20 mg each!

Ah, well, I would recommend staying with 1 pill, so you won't mess up so easily.
Good luck tapering down! Hope the aza will kick in soon.