(Inspired by another question, "tweets of meta questions".)
As far as I can tell, the automatic Twitter tweets of foo.stackexchange.com posts are good for those who are looking for tweets about foo.stackexchange.com and for those who are looking for tweets about a topic. Tweets about meta.foo.stackexchange.com posts, though, are only good for the former group, and #hashtags are aimed at the latter group. Therefore, I suggest tweets about meta.foo.stackexchange.com posts lack #hashtags.
An exception, though, should be MSO, since MSO is not just a meta site about another site (not of interest to outsiders) but the primary site for discussing the Stack Exchange engine (of interest to outsiders).