I really think one of the key points here is not what you use in the scene, or the source of it, but if you try to take credit by passing it off as your own modelling or whatever, well that is wrong.  We shouldn't have to mention the provenance of every element of every scene that we make, unless we are keen to spread a good vendor's work, but we sure as heck should be quick to point out that 'so and so object in the scene was not made by me but by jane doe' if someone in their comment on your work specifically asks if you made the object...  Hope that makes sense. :-)