as said in the above post, headstock angle is simply personal preference.
As for building a neck, I found that to be easier than building the body. I don't know why but I felt much more under control while building the neck.
If you want a tune o matic bridge then you will need either a neck angle, or you can reccess the bridge into the body. Contrary to what has been said above, a neck nagle can be done just fine on a bolt on and is done all of the time.