At intermediate stage polish a few pieces to a high degree. At a higher stage unless you're sightreading it's annoying not to.
Good point, but if I may add a bit: A teacher of mine once suggested cultivating three categories of works-in-progress, that is, keeping pieces that fall into three stages of development.
First, fresh pieces. These are brand new, the kind that offer an unfamiliar world, untrod terrain, and feed curiosity.
Second, half-cooked ones. These have passed the sight-reading stage, the interpreter having developed an idea of the organization of phrases, structure, and climax. At this point, you know more or less what you want and work to find and ingrain the physical means to achieve it.
Third, repertoire pieces, or those to be polished and performed. At this point, the work is probably memorized, though you continue to practice and study from the score to deepen your knowledge and stave off inaccuracies. You work to more clearly define character, timing, and tone (all of which are addressed in the earlier stages, but at a much more rudimentary level).
For me, these categories not only helped me to clarify my process, but to maintain interest during a practice session. There is nothing worse than growing tired of a "category 3" piece after six months of labor, nor being robbed of the chance to perfect material (a danger in accompanying) by constantly remaining in category 1.
May your practicing be fruitful, interesting, and joyful!
Sincerely,
JBM