I don't do what people would call "fancy" or "pretty" builds and only once through necessity received "free" parts as I did ask when I found myself having to do a build and being waaaay short on the cash end to do it right.
I do think you owe the companies that sent these free parts some sort of acknowlegement as to not do so is essentially spitting in their faces.
Where I think this gets gray is when you drop your standards on the build just to get free parts.. IE: Use some POS PSU because it was free and then tout it to the heavens.
Then you've lost your credibility and your personal respect.
In my case I contacted the companies for the same parts I'd have bought myself, IE: The best for the build.
I was lucky that the companies helped me out but if they didn't I'd have delayed the build rather than put junk into it.