Here's a good litmus test for a company we're considering joining: do at least their engineers (if not their PowerPoint+Excel biz people) use Linux for their laptops, not only for their servers? If they do use Linux, I'm just going to assert that they're likely more clueful than average.
Similar with startup hiring. Two options for this: (1) give a resume-screening boost to people who seem to have bought into Linux; or (2) announce in your job posts that the company pragmatically uses Linux for everything, to attract people who see that as clueful, and scare away a lot of others.
(Unfortunately, #2 also alienates some mostly-clueful people who really like Macs, and maybe even some clueful people who, through some cruel accident of history and gaming rigs, only know how to do Windows.)
(Actually, even more than Linux, I suspect that a startup using a BSD would probably be more technically clueful than the average Linux shop. Because some fringe tech seems to attracts the smartest and/or best-motivated techies disproportionately. But I'd say Linux is a sweeter spot overall for more startups.)