Even where SOX is not required, it is well on the way to being considered a "best practice." If you're not doing the things required by SOX, they'll probably be recommended. The procedures in use by most auditing firms have been adjusted to reflect SOX. Remember, Arthur Anderson went from being one of the largest and most successful accounting firms in the world to being out of business in a few months. That left a big impression on the senior people in all accounting firms.
Additionally, the auditors who audit non-SOX clients also audit SOX clients. At this point, most auditors probably audit more SOX clients than non-SOX clients. It's very hard for some (if not most) auditors to shift gears and adjust their thinking when going back and forth between SOX and non-SOX clients.