What issues might arise if you sculpt proportional changes just in the multires?
Hi ffearguyq ,
The important thing here is to sculpt on a low level of the Multires, so as to not lose all the fine sculpted details. With those broad proportional changes it is easiest to make them on the Base Mesh. In principle, you could also sculpt on the Multires with the sculpt level set to 0, that would be the same as using Scuplt Base Mesh, with the difference, that with Sculpt Base Mesh, you can see the higher subdivided Mesh, while only affecting the Base Mesh.
Generally you always want to keep basemesh and sculpted data spatially in sync. Changes to the base mesh directly affect the sculpt, but changes to the sculpt do not affect the basemesh. So your multires sculpt data can become disjointed from the base like this gif shows:
Which can be a big problem, like if you wanted to export your base mesh to another 3D app and your sculpt data as a displacement map.
The Sculpt Base Mesh toggle allows us to make large proportional changes to both basemesh and hi-res sculpt data at the same time, keeping both in sync.