Nope, you should think up a plan of how to come back, no matter how improbable it may seem to you at the time. If the other team hasn't ended it yet, short of you having two or more inhibitors down, forget it. And if they still can't end it then, yes, you have a chance, because the power gap closes if the game lasts long enough. And it means they suck at endgame and lack the teamwork to do it, indicating that you absolutely have a chance if you play smart enough, and keep your team morale up.
I learned this after a comeback from 1-15 late in season four. We still won, fourteen kills down. Never forgot it. Considering that the first few are by far the most important (if only for team morale, the most important thing in any game, regardless of elo, which I'm starting to realize is mostly meaningless until above Platinum), that gave me eternal hope for every game from then on.
So, I never surrender. Ever. You deal with that however you need to, because I support always voting no in Ranked (practice-mode unranked gets a pass, that's a "whatever" thing). I will vote no every single time, no matter how behind the team thinks we are, because the fact is, the longer the game goes on, the more the power gap invariably evens out. That's simply how the game works.
Besides, if they have the time to keep trying, by goodness, you have the time to think up a plan instead of thinking of how over it is. Everyone's got the same thing they want and the same risks on the line. At the very least, put up a fight and make it hard for the enemy team, if that's what's important to you. People who vote surrender multiple times and start trolling their teammates, really, are also partially why I vote no, because a good amount of the time, we end up winning, anyway, despite that one guy who insists it's over. Then I can say "I told you so". Even if we lose, at least some of us tried. I don't care if I have to go General Zod on the enemy carry if I'm the last person standing at the end of teamfights, I'll fight to the last breath of that Nexus, and not lose focus.
It's all mentality, man, in seriousness. If you think you're going to lose, you probably will make it happen on your own, and sabotage your own chances of winning. It makes no difference how ahead they are if the game isn't over. It's that simple, and I will forever stand by that. I've been told that's trolling (it literally isn't, look up what that means, in even the colloquial sense. "Didn't give up" is not a reportable offense, even if I've been reported for it.). I've then radio-silenced those people, thought "Deal with it or go afk for all I care, this is a part of the game, and I don't expect you to thank me if we win", and continued whatever plan of comeback is even remotely possible at that point.
At the absolute very least, I'll keep the replay (I auto-record them) and watch it to see exactly where things went wrong and learn from it until my next game. I always analyze a loss and see where I could have done something different.
Never give up. Never surrender. Watch Galaxy Quest.
