I have read that they need some "nurse branches" to remain for best results (branches with leaves).
I'm pretty aggressive with topworking the annonas, and on a recent trip to a nursery, I watched them graft a batch, where they stumped them extra low and cleft grafted. They claimed 95% or better success.
My favorite cherimoya variety is Pierce, but I also like El Bumpo, Sabor, and many others with exotic aromas.
As far a vertical shoots are concerned, aggressive pruning will cause the tree to make thick vertical shoots. I like to find branches slightly less that 60* from vertical if they are yet to harden off(achieve 3/8" diameter), but if they are already thick, I like to pick the 60* crotch angles, graft to those branches, leaving the dominant buds facing up, so that way when the graft shoots new growth, I can train the branches outward.
Either way, 60* angles are key due to the strength of the branch connection. You want a strong connection when a branch can hold 20+ pounds of fruit.