All depends. Do you know if you have a clean-out elsewhere? Generally they're very close to where the main drain leaves the house, not the middle of a floor. Is there a smaller drain nearby? If so you've got an abandoned toilet & shower. If you have no idea, I'd err on the side of caution and leave some access to this. I've seen cleanouts go straight down, Tee, Y, or take an immediate 45º. Check your utility room for a big drain with a Y shaped pipe with a threaded cap on it. Honestly I doubt they'd put the only cleanout in the middle of a floor.