Rob,
I think alot depends upon what's in close proximity to the fairway.
OB, Water, Woods, etc., etc..
I also believe that the length of the hole may have a bearing on the appropriate width.
You can't view fairway width in purely linear terms as a fairway could be 50 yards wide, but due to the sloping terrain, for all practical purposes, only 20-30 yards wide.
Shinnecock and WFW would be good examples of holes where fairway width isn't what you see and measure.