The moss is almost a must for Shohin, because as Daan points out, the tree easily looks newly repotted i.e.
There are exceptions though, I think. Some trees may give you an idea of their living place and conditions, that make it unnatural with a neat moss covered surface. To my taste, there can be variations that will work well without a fully covered moss surface.
A tree with deadwood will work well with a soil not fully covered with fresh mosses. Other trees, like a Satsuki Azalea with flower, invites to show a green soil surface of mosses that underlines the freshness of the tree.
In the end it is about the expression and feeling of the display and tree the artist wants to show. As long as the shown soil or mosses i.e. do not look unnatural, it is an artistic choice I think.
I have started another Shohin-freak issue about displaying, for Daan and others that might have some comments about that. Maybe others find it interesting too.