Good question. I'm pretty new to voluntarism, so I'm not authority (I'd love to hear what Hive account@lukestokes has to say on this), but my understanding is that anarchist are first and foremost opposed to all forms of physical coercion. Since reputation systems are not physical and arguably not coercion (after all, we should all be able to associate only with folks we want, and avoid associating with folks we don't), I believe they'd be considered acceptable.