Most, though not all, guidelines give you information on what the editor is looking for, in terms of genre, style, originality, target audience, etc. Take this information to heart. If you can tell that your story is not what they’re looking for, don’t send it. It will waste their times and yours, and gets you noticed—but not in a good way.
All guidelines—and I challenge you to find guidelines that don’t—ask that you familiarize yourself with their publication, to give yourself an idea of the kind of stuff they print. This is easier demanded than done, but many of these markets do offer free downloadable samples. Reading your markets really helps you get a sense of their style, preferences, tastes, and helps you estimate whether your own story might fit there.
(Personally, I hardly ever do this, mainly for practical reasons. Who knows how often this has hurt my chances?)
