Generally, it is appropriate to create soft edges in paintings. Because I always had trouble deciding just where to put soft edges in my work, I once asked someone on Wet Canvas to explain their rationale for placing soft edges in the locations they did in their paintings.
The answer was that soft edges could be appropriately placed in nearly any area where there were "shared values". That means that wherever there are either light values abutting one another, or darks values abutting one another, you could effectively, and appropriately place soft edges.
Soft edges can also be placed at the edges of any subjects that seem to be "turning" toward the background. That means that even if the background is dark, and the subject is light, you can create a soft edge wherever the two meet, appropriately.
However, I have often seen a portrait painter purposely smear the tip of a nose into an adjacent cheek of the subject, and that operation STILL puzzles me, because the painter seems to have no true, reason for doing so, and can offer none, other than to make the painted subject appear to be "spontaneous", or "flamboyant", and they can't seem to offer an explanation of their actions.
Other than those two reasons, I have no explanation regarding just why that sort of a flippant softening of an edge is considered an appropriate thing to do.