Main rig is currently sitting besides the desk on the floor, no carpet. Done that way so I can minimise all noise from it as I tinker with it. It would normally be under the desk, on a raised section of the desk, but currently out as I continue to reduce noise produced from it.
Rig in the Living Room is on the floor on carpet, but it doesn't have fans on the bottom other than for the PSU and that's a removable filter, and doesn't get as much use these days. So it's not a major deal on that to be on the floor.
Used to have rigs sitting on the desk, but they were much smaller back then (AT Cases, early ATX Cases with minimal room for cooling as it wasn't needed), and the desk wasn't as full of large items (27"+ Monitors, more like 15" Triniton Sony Monitor, so...) that would make it impossible to place rigs on the desk these days (unless if I got a much larger desk, which I don't have room for at the moment).
Also, none of my rigs have RGB (well except for the H100i original, but it's really only used for cooling, not lighting), so they can be placed anywhere.