The distinction between "published" and "preprint" is fuzzy and not all that relevant. I propose to adopt the  [same policy](https://meta.mathoverflow.net/a/942/) for both that is: 

>[W]e should avoid discussing  <strike>preprints</strike> papers  in general terms. If there is a specific question about a specific step in a proof and suitable context is given so that reading the paper is not a prerequisite, then it is on topic.