I look at it a different way in soem ways. If the model is the focal point of the picture then credit should be given. Lets say you download the polish fighter and make a scene with it. Since the fighter it the main point of the picture credit should be given. But lets say the mechanic working on the plane in the scene has a hat you downloaded. the hat is not the focal point of the picture but an additional thing so I feel it is not as necssesary. I would put items on freestuff but I lack webhosting.

The wit of a misplaced ex-patriot.
I cheated on my metaphysics exam by looking into the soul of the person next to me.