[Curtis]

Each fly line is rated according to the weight of the line itself. Accordingly, the rod manufacturers have designed rods to cast that line weight. Now, a slow action rod will load up under the weight of the line quicker then a stiffer rod but they are intended to cast specific lines.

I think it important to use lines that the rod is intended to cast. While you can cast a six weight line with a four weight rod, you won't be able to get much distance out of it as it will overload your rod very quickly and will not give you the performance it was intended to give with the proper line.

A good quality line is also very important. It will float higher, shoot further and be more consistent in it's makeup then a cheapo.