I don't think there's anything wrong with /ff and moving on to the next game if you're losing every lane and losing on all objectives. At 20, if we're down 4-6 towers, we haven't taken one of their towers and the score is something like 12 vs 4 then I say move on to the next game. Sure, they could throw, you could come back but it isn't likely. How much time is wasted turtling and dragging a game out an extra 10-15 mins just to lose? Just so maybe 1 out of every 10 games you can come back and get a win? That's 9 games wasting an extra 10-15 mins you didn't have to. Could've played 2-3 complete games with a chance to win w/ the time you wasted losing.
I think it just depends on how unlikely the comeback is. If you got at least one person doing well and it looks like they could turn things around then give them a chance to do their thing and make something happen but if everyone on your team has a negative K/D/A and you're losing on all the objectives w/ no map control, hang it up. Stop wasting time.