Ideally, you're listing credits the editor has heard of. For SF/F, I could list Asimov's (I wish!), Analog (yeah, right), and Realms of Fantasy (actually, I can list that one.) Most editors in the genre know these magazines, and sales will count for something.
If I list credits they've never heard of, one of two things will happen. They'll either ignore 'em, or else they'll take a moment to Google 'em (if they're bored that day.)
If a Google result pulls up a respectible publication, that could still help you. If all you're listing is a few unpaid, low-circulation, fanzine type publications, that's probably not going to help at all.
And I've been told that listing lots of unpaid publications (or vanity types) in a cover letter can actually hurt, as it tends to mark you as more of an amateur. I wouldn't list a Publish America novel as a credit, for instance.