The whole point of the Zone System was to give the photographer creative control of the process, and allow the photographer to interpret the scene as previsualized. You could have intended to have Zone III snow to create a certain visual effect, or you could have been going for an N+5 expansion to increase local contrast in the snow in flat light. Or maybe you only have a huge stash of grade 6 in your favorite discontinued paper. Don't let anyone second-guess your artistic intent or integrity.

Besides, I'm sure we've all missed a few stops along the way.