Whatever seems best, I guess?
I think that for the incised carving (engraving, technically, but most here will argue against that until the cows come home) it's best to be pretty close to final grit and have everything cross checked for tool marks so you don't mess up your lines sanding out any surface imperfections. That gives you zero room for error with your line carving, though.
For the relieved carving, you can do it at any point after the shape is filed in pretty well, just leave enough wood to work with in the areas you plan to carve. You may have handling dings and handling filth to sand out of the rest of it by the time you're through carving, anyway.
