Personally, I've used a self-star as a tiebreaker to call out better projects. If two of my projects both have N stars, and I think one is clearly better than the other, I'll star it so that it ranks over the other one in my profile page. This is especially important if I have many zero-star repositories, because some of them may not appear on my profile page at all (since GitHub limits the list to 5 repos). I might have a zero-star repository that I'd love people to see, but it could be buried in obscurity under a bunch of other zero-star repos.
I personally think it's reasonable to use a self-star to call out what you believe is your best work, especially since the primary place a project's star count is relevant is in ranking your own projects on your profile page.
Does that justification make it currently socially acceptable? I certainly won't judge you; I can't really say whether anybody else will. Anecdotally, though, self-stars appear to be good enough for Linus Torvalds, anyway. (At the time of this writing, he has starred exactly two repos, and they're both his.) Note, however, this may simply be an artifact of when GitHub first introduced stars and automatically converted all "Watch" relationships to stars. See SztupY's answer for more information.