I just saw a question edited from having the former tag to having the latter 3 tags. As the question only had two other tags, it kinda made sense. But before I start having misconceptions on the situation, I thought it'd be mighty better to ask. When it comes to a topic that covers only one or two sites, it is obvious to just to tag that specific site.
When you are referring to something on all of the sites, though, is it more preferable to use all of the individual tags versus the singular [so-family] tag? Is it more that [so-family] should be used when there isn't room for all the individual tags? Or are there connotations associated with the choice of one option over the other, so you need to mark them appropriately?