While I don't think it's required, giving credit where credit is due might be the better choice. If your game becomes viral, the recognition the creator of these free assets would get could help them out. But honestly, just do what you feel like.

If the license terms require attribution/credit, then you need to give it. Anything that is "public domain" or has a Creative Commons Zero (CC0) license does not require credit, but as @ffwied said, helping to spread the word about the people that create the assets you use for free is nice.