Always paid. Whether it's contributor copies, or cashy money, always paid. At a certain point, you have to say to yourself "I am a professional full-time writer. I am as professional as Stephen King. As Nora Roberts," and then do thou accordingly.
This doesn't mean I'm totally ruling out a story for free....but, if I do, it's for a friend, it's a favor. Even then, my schedule would have to be significantly more deadline-free than mine is before I'd agree to it.
Even if it's not much pay, go for the pay. It's less about the money and more about the credit, and the professionalism.