At one time, Dreamstime had said that they were factoring in the acceptance rate in search position, but I don't remember any other agency saying they were doing that. They don't generally talk much about how they arrive at search position though, so it's not likely you'd ever know if they did.
It always seemed like a truly daft idea to me to factor in acceptance rate (and not only because at that time DT was rejecting images for having a model release which seemed even crazier). From the buyer's point of view, they want to see good results and they don't care who produced them or all the other images that didn't get accepted. So wrecking the buyer experience for some internal reward system seemed completely misguided.