15x7 should be fine. I run 15x8's on my 37" MT/R's. The 15x10's were nice for tire wear (much more even tire wear) but not so good for rock rash on the wheels. I like the 8's because the tire gets a little taller and narrower and the bead is tougher to peel off the rim when aired down. Downside to 8's is tire wear isn't as even.
