Most people don't care as long as you cite your sources and don't try to claim that were done by you. Yes, there have been people that do that.
Most of the time people ignore citing sources too unless people are trying to claim it is their work. Though when in doubt cite the source... it'll potentially save you some headaches from the people helping out steemit by cleaning that stuff if they happen to see one of your posts on one of their OFF days. We're all human.
I do try to cite sources every time, even in comments.
You should be never flagged for that then. :)