I collect freebies, but only if they don't have restrictive licences, not that I do commercial renders, unless contests count, and I have no idea if they do. It is hard enough to keep a runtime organized with out worrying what can be used under what circumstances. 
Listing the freebies comes down to I'm not very good yet and wouldn't want to make someones effort look like crap and then blame or give them credit for something I made look crappy.
That said, since I became aware of VSS skin shaders I almost always use them, be crazy not to. 

So would you want people to credit you even if they(me) are still learning?