Do you ever soften or harden the normals of the geometry on a per-need basis, or do you have a set of guidelines?

For the normals on the vertices... So usually what I tend to do is I try to have everything soft and have a chamfer or a bevel on all hard edges. So yeah, that's how I usually work; I don't leave a lot of hard edges. But of course, sometimes you need to do it for certain reasons. And then you can just select all the edges, harden them and leave them in that state.

An edge case for having a hard edge amongst custom normals is when you've got a super thin face. Actually, I can show it with the shotgun here, because this is super thin. I haven't actually beveled the edge on the inside, because it doesn't need to be. It's just a bit where I can save some memory.