Joe,
I always believed that rod manufactures underlined there rods/blanks to protect there warranty. That's just my two cents.If you give a rod to a novice that loads a rod taper well into the blank it could fracture if to much line is out past the point of recovery.One time many years ago I watched a very well known flycaster at a sports show throw a line across the casting pool, every one was impressed with the distance and ease.I later found out the rod was a 7 wt. and line 9wt.
How's that for an allusion.
As for myself i always go at least one number up on a fast taper blank/rod.