In the past, I think they erred on the cautious side and told learners to always use Sie, and don't bother with "du" because you will never get to know any Germans well enough to address them in the familiar form (= "your German will never be good enough to get to know people that well in a German speaking way" which was probably true in traditional language classes ...).
But the world has changed. The internet has changed everything, for one thing.
And even the Germans are a little more relaxed about this kind of thing, especially among young people.
In the private German class that I go to, which has a very relaxed, but reasonably high standard of German, we use Du and Sie interchangeably, with each other and to the teacher (with her consent), because a) she doesn't think it's that important in this context b) it means we get regular practice in both forms. We know enough such that we'd be a little more careful in Germany, of course. But it means we can switch from one form to another with reasonable confidence.
Come to think, that's pretty much how it is with SSiW.