Mine sits on a low coffee table that I converted over to keep the rack off the floor. I have used them right on the ground, and it was carpet. No real issues with it except it was really low to the ground for pulling out the bottom tub.
Hang on a sec...taking some pics...be up in a bit.
Here's my RBI Rack on the coffee table I picked up at Target for $20.
Here's how I set up each tub.
Also, I realize I lied earlier. I only have five holes in each end of my tubs...not sure why I thought I had six.