I don't even use tiers, in my opinion they're kind of... dumb? Not dumb, but yeah. Unofficial, a lot of people won't even know what those are. Besides, when battling on random WiFi, everyone just uses OU with the occassional NU Japanese masterminds (ran into some of them in 4th and 5th gen - encountered a Starmie with the most unexpected and annoying moveset, a Hitmonchan swept the floor with me, got defeated by a Drowzee or Hypno, the likes)
If I were to take my older teams, they were almost always a mix of OU, UU and NU. RU didn't exist yet I believe. (or it did, but it was still a fresh tier) I just use the mons I like. My XY team is made out of the more OU folks, I like them, but they aren't my favourites, I made a team to win but I am not sure if I am happy with it.