The seat/bars support the weight of the rider, so I can't see a problem storing it upside down. I usually place a chunk of ethafoam under the seat and bars when I do this. Problem is, you are still occupying the same amont of floor space either way, plus you need more room to flip it (always a challenge when the SO's crap-knack collection is nearby).

If your ceiling is sloped, why not hang it by one wheel? My bikes spend most of the summer hung that way.