We all know how to decide weather a rod fits our purpose, but many of us don't know what line to use. It can be more of a factor than many of us understantd. Don't worry folks, I'll tell you why
Under most cirucmstances, I don't have a problem with having an over weight line on your rod. It will ABSOLUTELY allow you to cast farther than you would with a line built for your rod. (to a point) The problem, is fly presentation.
The problem with over weighting your line is not casting, or accuracy, but overall presentation. Immagine, for a moement, two fishermen on a trophy lake in Monatana. They both use a 4wt rod. One is useing a 5 wt line and the other a 3. They are fishing identical points, on identical tracts of shoreline, with indistinguishable flies they bought that morning at the same shop. The weather is warm and the wind is nonexistant. There is a minor hatch of caddis occuring on the lake they have selected, and they are both using a beaver hair caddis fly, because it is the most effective they have.
There are advantages to both options. The overweighted rod will cast farther, and won't sacrifice much accuracy. The heavier line will cover more water with the fly, and can present his line, in theory, to more fish, than his underweight counterpart. The underweight line creates less disturbance on the water when he presents his fly. Fish may be spooked by the presentation, but a smaller disturbance is going to have the fish fleeing a lesser distance than his overweight counterpart. He will present his fly to fewer fish, but when they flee from his surface disturbance, they may still be in range to attack his fly, where his partner may drive his quarry away.
It all depends on weather and the waters that you are fishing. Fishing a lake on a calm day, I'd choose a line weighted for your rod, or an under weight line, rather than one overweighted. On a day where the wind is blowing, you'll be able to cast more effectively against it, and the disturbance created by the overweight line will be lost in the surface clammor,and the fish won't mind. On a river, where the water is constatntly moving, the line is less of a consideration. If the surface is broken before you get there, use what you like. If the surface is calm, use the lightest line you're comofortable with. What you're comfortable casting with becomes the real consideration when the water is undisturbed.
Lots of people here have given losts of opinions. Most, however, I suspect have nenver tested most aspects of the sistuation. Personally, I use a 6wt rod, and have three 6wt lines in my vest, 2 8 wt, and 2 4 wt. Situation dictates wihich one I use.
Don't get stuck. Experiment. You'll have a better time if you can figure which one is best for the conditions you find yourself confronting. Calm air and water can decide weather you catch your limit, or nothing if you can't addapt to the waters you frace.
I live in an area, where fishing isn't difficlut. I've fishied from Canada to Colorado, Idaho, to Nevada , to Nebraska, and I know the waters I fish. I can't say, for sure, that what I have told you works in Califironia, or New Jersey, as well as it does in Wyoming, but where I fish, it CAN BE as important as the fly you use. Spend a day on the Miricle Mile with me, and say it an't so. Point I'm making, in my arrogant way, is that you have to be flexable.
Sorry for my rant. Half a bottle of single malt doesn't help, but I think I'd think I'm right even wen I'm sober.