I would pretty much never tip at a spin class in the US.
But I did tip the spin class leader when I was at a resort in Mexico. The class wasn't that great, but she didn't laugh at me when I tried to speak Spanish to her, so that was worth something.
When I was in Las Vegas, a place whose economy is even more imploded than Mexico's, I noticed that the towel dude at the casino's gym had a statistics textbook with him. And he was nice so I gave him a dollar. But normally I wouldn't tip them either in the US.