A builder told me that he would not use 1x8 hardwood flooring because of it's tendency to cup. He only uses 1x6 as his maximum width. I like the look of 1x8 better than 1x6. Am I asking for trouble if I go with 1x8?
The only way you could use 8" is if you cut about 3 or 4 v groves in the back side to help minimize the cupping. But even then, I would recommend face nailing or pegging.
I tend to agree with your builder that 6" is wide enough, unless you can find an engineered flooring that wide.