First of all, let me make this perfectly clear, I usually advocate staying with the manufacturer's suggested line recommendations or, in the case of the heavier weights 6-9, underlining by one weight depending on the circumstances. Wait a minute… did I say underline? Yes I did. Doug Macnair opened my eyes to this concept. You see, as Doug says, it's in the amount of line you aerialize in making the cast. Consider for a moment that while you and I now know the weight in grains for the first 30-feet of our line, we don't know the weight of the remainder that typically includes the rest of the head, its rear taper, and the running line ... An interesting thought? It should be, because for every ten to fifteen feet we add to the initial 30 hanging in the air, we effectively add another line weight to the load carried by the rod. Said another way: a 5-weight line becomes the equivalent of a 6-weight when 40-feet of line are aerialized, a 7-weight with 60 to 65-feet in the air, etc. At some point, of course, our 5-weight rod will overload resulting in either a collapsed cast or worse, a broken rod. Then, as Doug would say, the Ancient Fishing Gods will be laughing.
So... You should be fine going with your 5wt line.
