Personally I would tell them, I mean on my chat groups and so forth. I am not against resteeming, I'm against making it a condition. If I see a really interesting initiative I want people to know it, true, but for example I plan on posting ideas on trading and investment here in steemit... and most likely the people who will follow me are the people who are interested in those ideas (and many bots searching for upvotes). Resteeming is posting something on my feed for all my followers, and if they're interested in contests, they're sure to look at the contest feed and besides think on how you would feel by receiving tons of the same contest on your feed?. Maybe once Steemit changes how that works and shows the post only once no matter how often the people you follow resteem it it will be different, but right now I don't like forcing people to resteem in order to participate in a contest, it's just my opinion, though.