I did something similar a couple of months back, UA IAD-FCO then IT FCO-ATH. Didn't have any issues across the board, and flight was perfectly fine. No issues with interline bag, and I had about 14 hours between my arrival into FCO and departure to ATH. I wouldn't go out of my way to do it, since I can't credit the IT flight to UA, but if push comes to shove it's perfectly acceptable.