I understand that he's a gun for hire and has no side, but there are bounty hunters, and there are bounty hunters.
My impression of Fett is that he's known to be an unscrupulous guy, and I can't imagine the rebels trusting him.
What I could imagine is the rebels handing Fett false information regarding their plans, knowing he'd double cross them, thus giving them an advantage in the attack.
Don't know how that would work in an edit though.