Question about descending

Hello everybody, A320 question here. If I’m told by ATC to descend to a certain FL or altitude when approaching or already flying a STAR, what i'm supposed to do? Better descending with DES (so respecting constraints and VNAV and maybe descending at a low V/S or better pulling OPEN DES for a faster descent to the ordered altitude? Basically i have to obey ATC as fast as i can or i have some "room"? (Always done 1st yet) Thanks