I would say it depends on the type of wheeling you do also. If you intend of getting it to flex hard from time to time, you may consider a 10. I was running 12.5's on an 8' rim and I never had problems.
From what a truck parts dealership told me. "A 10 will fit better on a 12.5' tire." Maybe he just wanted me to buy the more expensive rim. Musta worked. I went with the 10 =D
Your better off with an 8" wheel. The bead holds better when you lower inflation pressure and you get more
height from your tire. I`ve seen a whole lot more 10" wheels pop their beads off-road than 8" wheels.