I'm a run what you brung kind of fisherman.
Many of those rods often behave more like an x/y/z-weight. I have a fiberglass Fenwick in a 2-piece, 8- weight that is the flexiest, mushiest thing I've ever casted. If yours has similar action, I can't imagine you'd notice a difference with one weight lighter line, especially on a small to middling stream, but I bet it would absolutely chuck a one weight heavier line.
A rule of thumb someone once remarked on tippet was to go no greater than 2x the weight of the rod. That said, I've been known to run 30# on an 8 to drift home-cured roe for cohos. Fishing bait with a fly rod generally shows you who the elitists on stream are immediately, too.