Questions about usage in different geographical areas are going to come up a lot. Plenty of answers are going to have to distinguish between standard usage on one continent versus another. Is there anything about this that we want to standardize? How do we want to tag it? Or is it too early to plan for and we should see how things shake out?
I think if the questioner specifically wants to know about the usage in one region (a tourist from one Anglophone country to another?), it may be worth mentioning in the question, but we probably don't need tags as a general rule.
On the other hand, it is good for those giving answers or usage advice to specify what variety of English they are most familiar with (and of course, if they're familiar with more than one, to point out differences on the topic in question). On the group alt.usage.english (as well as elsewhere), the abbreviations AmE, BrE, AusE etc. are common for this purpose.
I'm not sure if there's much to "standardise" per se (well, depeding on what you meant by that).
But yes, it's an important issue. It's clear that different varieties of English will need to peacefully co-exist on the site – both in the sense that questions and answers can be specific to some varieties and in the sense that different contributors will themselves employ different dialects (well, at least different spellings of certain words) in their posts.
It certainly will be useful to tag questions with something like
australian-english (or even something more specific like
boston-english if need arises), but of course only when the question really is specific to that particular variety of English.
Going by the list of dialects of English in Wikipedia, there are a lot of dialects of English. That would be a lot of tags.