I ran across a giant along the road once and he didn't attack me either (nor did I attack him.) I think they will go hostile if you kill their mammoths, OR enter their camps. Since this guy I encountered was not near a camp, I guess he was basically "I won't giant-stomp you, if you don't shoot an arrow at me."
Giants on their own in the wilderness do not attack you even if you're standing right next to them. They only attack if you enter their camps or go near their mammoths. I've never had one aid me in battle.
Thats why i don't like bounty's to kill giants there no problem at all as long as you don't mess with them they don't mess with you.
Walking for a bit with the giants is part my pc everyday
So basically,they are friendly,but if you attack their mammoth or go near their camps they go crazy?
This however is different for the cows I sometimes see them with.For me at least.