Hey Tom.
I have built a series of jigs to help me with scarf joints. One for routing the scarf bevel into the neck and headstock pieces, one other for gluing it together, and one for thinning the headstock after it's glued up.
Scarf bevel jig:
Pretty simple sled jig. I set mine at 15 degrees.
Gluing jig:
You apply glue to the mating surfaces, clamp them firmly to the jig to immobilize them, and then clamp them together. The vertical stays are glued to the pine base board, and then screwed in from behind.
Headstock thicknessing jig:
Also quite simple. A router with a riser plate removes the extra stock. The angled piece is actually a piece of alder...a cut-off from a body I made a few years ago. It's glued to the birch ply base plate and then screwed in from behind.
It's the most repeatable and reliable system I've come up with for this process. The biggest downside is that if I want to use something other than 15 degrees, I have to make a whole new set of jigs....which I am currently doing for 12.5 degrees. The only other downside I've experienced is that I have to make the neck blank several inches longer, since I cut the headstock portion of square, rather than attempt to use a table saw jig to cut the 15 degree taper (because I don't have a table saw).
Edit:
It seems like it would be relatively simple to make all three of these jigs adjustable for angle. But...I don't know that I care to make the effort, as I don't change the angle often.