As much as I dislike that steemit has a structure where this advice does sound a little like telling peasants to not bother the nobility, I think it is pragmatically worthwhile. I've seen enough people pick fights they can't win to know that you need to think really carefully about this.
That said, there is a lot of content that unambiguously deserves to be flagged - spam, plagiarism, tricking people into voting for you etc. People don't have to flag this themselves, but should report it to @steemcleaners. That way you can do something for the good of the network, without risking your own earnings or reputation.
Yeah, I flag people occasionally. But some spammers, plagiarists and scammers literally won't take notice of anything else, and I like the idea that I can gently encourage people (with my tiny, tiny down-vote) to change their ways before they get hammered by a much bigger account.