That's a good question, but I have to think about the answer. I guess I have a personal expectation for the level of rendering. Some elements of the painting I know from experience to leave rough, because when combined with other more polished elements, creates a dichotomy that plays off against each other giving the painting a dynamic. In the case of this particular painting, it creates and illusion of depth of field.
I have seen many artists struggle with the exact opposite; they obsessively smooth out all roughness, and thereby lose any life and spontaneity might have.
How do you do it? Experience, the trust in yourself to let yourself go and just go with the flow, by not expecting a masterpiece and as Bob Ross used to say, "There are no mistakes, only happy accidents."
=)
I've been thinking about this reply a lot and it's interesting in how it interacts with my day job, which involves a lot of technical writing.
In writing there are always spots that are rougher than others and I suppose that as in art, part of mastery is in learning to both recognize those spots and know how to fix them. A large difference is that if a spot is left rough in writing, it's usually a matter of priority or deadlines. It's very interesting to me that roughness can be intentional, either as a deliberate stopping point, or as you point out, as technique for contrast.
Which isn't to say that all good writing is infinitely smoothed. In the same way that you've seen art lose its spontaneity, I've seen overly polished writing lose its life too and either become plodding or soulless.