Never wise to limit your audience when you don't have to.
Except that if you go too far the other way, you have a real danger losing your audience on the other side. No one wants to be lectured about stuff they already know, especially as slithering in the data often slows down the story.
On the whole, the best way to slither it in, to please both possible audiences, is to disguise it and give it a dual purpose. It's not quite foreshadowing, you can get away sometimes with something that just expound information that the reader needs to know. But it's best to give a dual purpose: characterization, plot, etc, and then expound. Preferably after you've made the reader curious, but if you slip it in subtly enough, the reader might not notice the lecture. Even the lecture on what he already knows.
It also helps if the style is smooth and elegant. Flat-footed exposition is worse than most forms of flat-footed prose.
And, of course, sometimes you have to decide that yes, the readers should know that. You don't have to tell anyone that dragons are scaly. And if you did, you should wonder whether the reader is worth alienating those readers who would find being told that dragons are scaly condescending. On the other hand, I once had a character allude to the fact that only virgins would be safe in a certain forest, where she had seen a unicorn. This was, in fact, set-up, to alert any readers to the famous connection between virgins and unicorns. But I got a crit saying that surely I meant they wouldn't be safe.
I concluded that this person was -- not part of my target audience. Though partly because I suspected an unwillingness to suspend disbelief, which is very hard to get around with any sort of rhetoric. But since everyone else had had no problems, I suspected that if I had made it less subtle -- and hit the reader with a sledgehammer to get it through -- I would alienate readers.
