Yes, they touch the ground. Proper twerking requires a big butt and proper attire. According to some women I knew back then, there's a science to it. Even the underwear selection came into play according to them. The flesh of the ass needs to have travel above and beyond what the hips are doing. So, for lack of better imagery, some VERY skilled women could squat real low and basically dribble their butt cheeks on the ground. I've only seen one person do that live in front of me and everyone was losing their shit. It was incredible. She could make one cheek hit the floor then the other. Left right left right...like dribbling two basketball really fast.
EDIT: I guess I should clarify that the bounce gave the illusion that the cheeks were touching the ground, so as far as I'm concerned they were. From my angle they definitely looked like they were bouncing but they could have been a couple of inches from the ground. I didn't physically check as I was not an idiot with a death wish.