Many years ago, just to experiment, I took all my rods out and cast them each with all the line wts. I had. With modified technique it is possible to cast a 4wt with a 9wt line. All my rods would work when grossly overlined. Though I did this I don't think it's a good idea and can't recommend it. Severely overlining can damage a rod. Best idea is to start with the line wt on the rod. If the WF isn't satisfactory try a DT. Overlining by one wt will probably make it load better but it also might shorten your casting distance. It might affect line delicacy also. Usually better to evaluate your fishing requirements and test cast with several lines before deciding. Most rods will have one line weight that they seem to cast the best with.