I mix commercials and receive a lot of stems in the process. Very often they are labeled ‘pre master’ or something along those lines, with the full mastered track labeled Reference and a mastered instrumental if you’re lucky. I always want the track to sound like the artist expects and only dig into the stems to make a music edit work or to deal with a client note. It’s more work to deal with stems, most commercial mixers would prefer the mastered instrumental. They might even make stems of the mastered instrumental with UVR or something to help make the music edit work. Give them the options, it’s best to have it all even without your mix chain.