Start with the structure
Check thicknesses, clearances and connections. Different areas do different jobs, so no single number replaces professional judgment. It is also important to check junctions such as the head against the shank, and setting areas against the metal around them.
Check the stones as part of the piece
Make sure the stones suit the flow of the design, that the layout is right and that the setting structure makes sense. Where symmetry is required, check it explicitly; in an asymmetric design, check that the asymmetry is intentional.
Then check the integrity of the geometry
The model needs to be a valid solid for the production requirements. Open edges, non-solid bodies or unresolved joins can cause problems. Still, geometric validity is no substitute for correct jewelry design.
Go back to the original brief
Check that the model still meets the requirements the project started with: size, stone dimensions, target weight if one was set, proportions and design decisions.
Only then prepare the STL
The mesh is the stage at which geometry is passed into the production file. Choosing and checking it comes after the model itself has passed the earlier checks - not instead of them.