I got the Giant's promise to help at the Citadel, and then in keeping with my evil nature, killed every last one of them.
They still showed up at the Citadel anyway! I would think that their promise would be void if I slaughtered their chieftan and tribe...
Giants
Ha!
Somehow I doubt it...but it's not something amazingly important. Just means you get a few thousand more xp (300+ for each Giant, 400+ for the chief).