Since the valve guides typically extend into the port, re-shaping the guides is technically part of porting most engines. I've seen some that not only cut the guides down, but gave the port wall around the guide a nice airfoil shape to help flow around the remaining bit of the guide and the valve stem.
Heck, if we didn't need the valves to rotate (in order to move any hot spots around and even out any wear) we could have the stems be airfoil-shaped and get even better flow that way.
But again, that's primarily of help in high-flow situations. Which are found at high RPM, which is the opposite of where we drive.
-soD