I've heard of a lot of anglers that overline their rods, usually by one, but that has never seemed like a good idea to me (and Diver Dan confirmed my suspicions, I think). Aren't lines already more or less matched to the weight of rods they're designated for? I realize that some lines are already "overweighted" somewhat--I've got a Rio Grand on my 6-wt, which I understand is half a size heavier. Why would you overline a rod?
I was fishing in Belize earlier in March--my first time saltwater fly fishing--and used an 8-wt tropical line on my 8-wt. rod. It cast fine, even with a bit of wind. One of the guides and a number of the guests told me that a lot of saltwater anglers overline their rods--casting a 9-wt. line on an 8-wt. rod, 10-wt line on a 9, etc. They claimed overlining made the rods easier to cast, especially since saltwater fly rods tend to be stiffer and faster and you often cast in the wind a lot. That makes some sense, but I'm still a skeptic.
Anyway, didn't mean to hijack the thread, but I find the discussion on overlining interesting.