Darren
it's a guide as always, based on someone having built the blank into a rod
and tried it out during the R&D stage.
You may find you prefer a blank rated as a 5 wt as a 4 or a 6 wt , the same as any rod you try
if you can, try a rod already built on the blank. Just as if you were buying a rod.
There are no hard and fast rules, personal preference has a lot to do with it.
Otherwise trust the blank designer and take what is written, as correct and choose an appropriate blank.
Action is a bigger issue than line weight in many cases.
Try if you can , or take a gamble
I'm sure it will all be fun
sandy