Primitive Archer

it's better to go with a thicker handle than necessary and tiller bend into the handle area later in the process. Have the handle bending too early and what usually happens later on is  this section bends too much through the handle, and not evenly enough in the limbs, when you reach your intended draw length.

+1

There is no absolute reason why it's better to round the back of a bow, or the belly for that matter. It's mainly a stylistic thing, or dealing with the natural crown of a narrow tree stave. You can go deeper into it, reasons like trapping, or crowned staves, and wood usually being stronger in tension than compression, theoretically advantages performance in some designs.

You can tiller the whole belly including the handle if necessary. It depends on the dimensions of the stave you start floor tillering, and the weight of the bow you intend. Unless you are familiar with the design, and species you might not have a good idea of how thick to go. In this case it's better to go with a thicker handle than necessary and tiller bend into the handle area later in the process. Have the handle bending too early and what usually happens later on is  this section bends too much through the handle, and not evenly enough in the limbs, when you reach your intended draw length.

If you start with known dimensions of a bow that bends through the handle with a good tiller, you probably won't need to remove much wood if any from the handle area.

This is out of sheer curiosity regarding bend through the handle bows . does the design of these bows sometimes require the face of the back of the bow to be slightly rounded ?
when tillering is the whole of the belly tillered including the handle ?

rounded back required would be only if you were replicating a specific design. bend thru the handle is a generalized description of many examples.

since the type is all "limb", the handle area needs to be tillered to bend properly.
having less bend in the handle area and more bend in the mid limbs works well

pellets bow shooting slowmotion
wrist twist.
https://www.youtube.com/shorts/cyagOi7bSfY

Thanks for that answer Mark , it makes sense .