In one of the episodes of Kuroko no Basket, Midorima said that his shots depend on his nails. Why do his shots depend on his nails?
In addition to being a perfectionist Midorima uses the tip of his fingers to direct all his strength to the ball. It is not easy to shoot the basketball at that range.
The generations of Miracles have incredible talent, but their bodies can't keep up with them. Just like Aomine and Kise who have additional strain on their ankles with those quick movements. Similarly, shooting the ball in such a high arc puts strain on his fingertips, and we all know how irritating it is to wait for a nail injury to heal.
Something similar happens in another sports shounen, Diamond no Ace, where a fastball pitcher hurts his nail while pitching because he was putting all his strength on his fingertips.
TL;DR Midorima puts all his strength into the fingertips whick strain his nails. Thus he constantly maintains them to be always on top of his game.
Midorima is displayed as an accurate shooter who knows all the factors that influence his shooting action. He's also a neatness freak. The two factors must cause him to trim his nails.
Midorima, as we all know, is a very superstitious character. He maintains his left hand especially because the Oha-Asa said so. He tries to do all the Oha-Asa says like his motto "Man Proposes, God Disposes" so he has the highest possible chance of success.