Given that the /group/ has to agree to it, RAW would state that there cannot be a single person who doesn't want the concession.
Though that assumes you have a group of people who are all reasonable all the time. And we all know how often that happens.
Sometimes you just don't want bad things to happen to your character, but the dice are against you, its late and you had a bad day and all good intentions go right out the window. That happens, and that's not a good thing, but to get the game moving again, the GM can invoke his role as the final arbiter and just make a ruling, if a consensus can't be reached. Or maybe a break is in order.
But in a well oiled gaming group, situations like that should be relatively few. Nearly anything is subject to negotiation, but taken out results and concessions are the main things, I think. If someone is not satisfied with the outcome, the concern should be stated as clearly as possible, so it can be changed accordingly. "I don't like it" isn't going to get you anywhere near a solution. Again, communication is key, like with any RPG really, but some of the mechanics of Fate actually require and encourage it.
