Why do I think people are overly preoccupied with this? Contemporary Italian, whether we like it or not, is rapidly moving toward use of the tu as a default, whereas historically the formal was the default. This was pointed out in a recent letter to Beppe Severgnini's blog in the Corriere della Sera, wherein a reader noted that the tu was busting out all over, and Beppe (a self-styled expert in the Italian language, and a number of other things as well) agreed.
Why shouldn't you fret about this? If you're fretting, it means you're a foreigner who knows Italian imperfectly. The Italians will know this, and will no more expect you to have mastered the subtleties of the hierarchical relations implicit in this usage than a Japanese will expect you to seamlessly perform their bowing protocols. Personally, having more than a little of the anarchist about me, I wish all these things would just go away.
And don't think for a minute that amongst the Italians all is well in the world of tu and lei. As the usage is indeed voluntary and subjective, Italians are perceived all the time as using the wrong form by other Italians. Usually this is done by some impertinent person who has failed to realize the social superiority or at least distance of the person addressed. Leading not rarely to the snippy: "mi dia del lei!" (use the lei form with me).
Feeling better now? (ti senti meglio adesso?) If you are still anxious, take my little test on approprate usage, here.