I have not owned any graphite rods that cast poorly with the line that the rod was rated for. Nor did they cast any better with a line size up or down. With today's graphite rods, I don't think a rod's line rating is just a best guess by the manufacturer. Those that feel they have to line up to get a fast action rod to cast better, IMHO need to either work on their casting, or perhaps may find a moderate to moderate fast action rod to me suitable to their casting style.

Although like Jeff when fishing tight streams, I have overlined a shorter rod to make the rod load quicker. But in those instances, I have very little fly line past the tip.