Plant crowding leads to environmental issues, mostly airflow related. It can create a real nice climate for molds and mildew, warm moist organic material with little airflow is a bad combo.
That said, it is a delicate balance as you want to use all space. I treat with some variety of fungicide as a preventative during veg and crowd them in, I count on pulling males to make room during flower. Once in flower and bud sites have formed, I evaluate all the plants and trim off any branching that looks less than productive or if one plant looks like it will be a stronger producer I may trim a weaker one back to give more room. End goal is to have all my colas a few inches apart, never let a bud rest against another bud or the wall.
