Not in a federated (server-to-server) manner: that part is purely opt-in as instances follow one another.

But clients do seed videos by default. You can opt-out of that in the video settings, and if your browser doesn't support WebRTC it will not seed anyway (simply download video over HTTPS).

I believe it is more simultaneous viewers that counts, and they start to upload as well as download while watching (and you can also opt-out of that).