It?s very simple.
You ask first. Then you don?t waist people time and it doesn?t appear like your taking advantage. There?s no middle ground because you don?t get to pull the puppy dog eyes and say, oh go on, after the fact.
Legally, I don?t know if anyone is particularly in the wrong but it?s always in good faith to ask. And it doesn?t matter if you give credit or whether you think it?s for the good of everyone, if it?s so awesome you present it to them and convince them to agree.