I read in one of my books that when a rod is listed for a certain weight line, its meant for WF line, and that the rod is meant for a weight lower for a DT line. For example, a 6 weight rod is meant for a WF6 line, or a DT5. Is this true?

The reason I am asking is, I have a 6 weight rod that I always wished was a 5weight. So, I was thinking about trying a DT5.