In practicality it depends on your dioscese and seminary.
For example, my Archdiocese (Seattle) allows the Theologians to wear blacks (But I dont think they can do cassocks unless it is liturgical, but I am unsure about that) while at their schools and only if their schools have that as the uniform (eg. Mt Angel guys waer blacks, Mundelein guys dont, and I have no clue about St Pat’s or St John’s in California but my suspicion is that they do not). Our theologians do not wear clerics while in the Archdiocese unless they are deacons.
By contrast, in Spokane (where I go to school). The Seminarians wear clerics at least when they are doing active ministry or somthing official (parish work, or events in which they are there as semianrians) as soon as they are accepted into Theology.
It just depends on the Bishop and what he wants, as he is in charge and knows the culture or specific situations in the diocese.
My personal thing with clerics is that owning the clothes is not enough, it is a reminder of simplicity as much as it is a public witness. It seems to me that once one adopts wearing blacks at least as a seminarian) one should also eliminate much of one’s wardrobe. Outside of underclothes, workout clothes, and a few casual shirts: Own the shirt on your back, one to three that are clean or being laundered, one more dressy shirt, 2 or 3 pairs of pants, and maybe* a cassock if you really have need of one and it isnt just vanity.**
I think we younger guys get all exited because blacks seem really cool to us, and that is a huge reason I think some bishops dont want us vainly parading around in them acting like somthing we are not, on the other hand the philosophy for why bishops who have their theologians wear them do that is to get the guys used to the idea that they represent somthing 100% of the time they have that collar on and need to behave accordingly. Again, it depends on the philosophy of the bishop and the needs of the local Church.