They do follow tiers, but the borders are a little different. Simply because certain pokemon (example: Bronzong) function MUCH better in a doubles setting since their bulk and function in support are more helpful. Also, because it's more fast paced, annoying problems like stealth rocks and entry hazards are less common. This makes practicing prediction a little less taxing. By example, look at the VGC teams being posted. Though there are lots of OU pokemon in them, a lot reach out to UU and even NU to pick up some unexpected gems.