I always catch the mascot with the Master Ball (except the Kanto games, where I catch Mewtwo with it instead).
And I think I didn't catch Necrozma with the Master Ball in US... Actually I'm not sure about Lunala in Pokémon Moon either, I'll have to check.
But yeah, I always use it for the mascot, especially since they started giving them away in the Lottery thingies and stuff like that. It's like, even if I use I usually still have one left. Not to mention all those games I exploited the Action Replay like there was no tomorrow.