This being a site about English language, we should be well-equipped to decide about something like this, eh? :)
Should we prefer singular or plural in tags, in cases where both would make sense?
As of September 5th, 2010, the usage is mixed (unsurprisingly).
On the more popular SE sites this is a well-known issue.
Edit: Obviously certain tags (e.g. grammar, syntax, writing-style, etymology, vocabulary, literature, etc.) should be in singular in any case, as plural just wouldn't make much sense—this question is not about such tags.