Rods are designed for use with weight forward lines. This is because the sale of weight forward lines is more than those of double taper lines.
If a rod has a single line rating ie 6#, that rod will be rated for a 6# weight forward line.
If you use the same rod with a double taper line, you could go down one weight.
For rods with dual line ratings, 6/7# for example, then that rod will perform best with a 6# double taper line and a 7# weight forward. The reason for this is that the weight of double taper line aerialised when casting will be slightly greater than that of a weight forward line as the double taper line has a thick belly whereas with the weight forward, the weight is in the head.
I use Scientific Anglers #6 Weight Forward (WF)Intermediate tip, Scientific Anglers a #6 WF Sinking tip, and a Cortland 555 WF floating (very nice) and a no-name #6 sinking.
Richard