Tempering wood is the process of wetting and drying the surface(s) of dry wood – that is, wood that is optimally dried to local and seasonal conditions and fit for cabinetmaking.
If in doubt as to how a veneered panel will eventually settle, pre-wetting the surface to be veneered (to simulate the application of water-based animal glue and the wet veneering process) and allowing the wood to dry out again, will give an indication as to the likelihood and extent of cupping or bowing.
One can then assess how much water should be used in the veneering process, or if the reverse should be wetted prior to veneering, or indeed, the reverse should be subjected to unreasonable heat to shrink that face.
I don't normally go through the trial process any more; I can usually gauge what's required depending on substrate, substrate thickness and veneer thickness.
As mentioned above, I welcome a certain amount of cupping/bowing when veneering mirrors etc., but, unless copying a vernacular piece of furniture – such as a chest of drawers – that exhibits bowing of the drawer fronts and/or the top, I take care to make veneered panels flat.
A prime example of necessitating flat veneered panels was a circa 1775 mahogany cabinet-on-chest I made some years ago. Warped door panels in this era of enlightened cabinetmaking would be unacceptable.