Having some historical knowledge of history and culture is always helpful to frame the works within their context. How overtly helpful that is will vary; for example, I found Aristophanes funnier when I understood what he was commenting on, and I find Latin poetry more interesting when I can see it within the context of a larger tradition, that sort of thing. That being said, I don't think it's impossible to enjoy a work without that.
If you get a reader meant for students, some will have introductions and notes that give you a summary of what's going on. If you want a general reader, this one has sections on various aspects of history and culture, although like any textbook, it isn't comprehensive. Sometimes if you look online, though, you can find things online that will give you information as well; sometimes classics departments even have things posted online, which is nice.
Here's Aristotle and Herodotus. Turns out Plutarch's isn't very extensive, and wasn't worth tracking down.
My modern sources (the one I have on hand is <em>The World of Athens</em>) don't really give anything more robust than is available in Herodotus and Thucydides.