Nobody wants to hear my opinion on this, but here it goes. You can overline and underline by one weight rating without doing harm to the rod. In fact, I have fished five weight plastic rods that are happier with a WF6F line. Oh, the blasphemy, I know. Now, since I fish cane more often than not, I will use everything from a WF2F to a WF4F on my 3wt cane rod depending upon wind, water conditions, presentation requirements, etc, and will not harm the rod. I don't know how many of you subscribe to the Common Cents system of rod measuring, but many of these so-called "fast" fly rods, are simply rated for a line weight that is WAY too light for the rod. A good example is the TFO IM6 (Signature Series) rods. The 9' 6wt rod is rated on CC as a 7.03wt. The manufacturers do this to advertise casting distance. I am sorry, but distance is a moot point if you can't hit a spot the size of a swimming pool.

Find whatever line (fly shops allow you to test lines on a rod normally) makes your rod and you happy, and slap it on there. Cast, fish, enjoy!