It really isnt hard to taper after you scarf and I think it may make lining up the scarf joint along the whole length harder if you taper first.

David

   IMHO, one would probably double the work if you taper then scarf. Provided you have room to work on the entire mast it's easier to have the entire length to index your tools on. 

Hey Jerry, should have thought more carefully before posting last comment, there's a larger issue at stake. Aligning the two parts after taper would take some interesting jigging to make sure the two sections were perfectly co-axial. You probably don't want your mast tilting off to one side or the other.