Before you can over/under line a rod you need to know what is the correct line weighting for the rod under normal circumstances. Most rods nowadays, especially the fast action rods, are severely underrated and an 8.5 foot rod labeled ‘5 weight’ is really a strong 6 weight/light 7 weight rod. So when you put a 6 weight line onto this rod for working normal distances you’re really not over lining it at all, you’re actually putting the correct line weight on the rod.
Rods are supposed to be rated with 30 feet of line. However, there are all kinds of different lines available. Rio Gold and SA General Presentation are lines that are actually ½ weight heavier than rated. Then you have different front/rear tapers, belly lengths, etc., and each of these variables affect how a rod.
Once you know the true line rating of the rod then you can over/under line depending on the rod’s action and fishing circumstances. True slow, medium and maybe some medium-fast action rods should be fished at the true rated line at distances under 40 feet. If you really need to start pushing distance then you underline because 40 feet of 5 weight line will probably weight the same as say 55 feet of 4 weight line and it is the weight of the line in the air that the rod needs to support and carry. This is why you don’t overline for distance casting – you’ll overload the rod, your loops will open, you’ll get poor energy transfer and the result will be you won’t be able to generate adequate line speed. Over lining one of these rods would help if you’re fishing a small pool say 15 feet long and you need to use a 9 foot leader so you only have 6 feet of line in the air. Then over lining could make sense with such a short amount of line in the air.
Most fast action rods on the market today (unless a top end fast action Sage, Winston, Scott, etc.,) are probably underrated so they benefit from over lining to help load the rod when casting under 30 feet or so. Beyond this distance it is probably best to use the true rated line because most fast action rods are designed to carry a lot of line in the air to maximize distance. The top end fast action rods, though, actually perform quite well with their rated line across the board.