It's sometimes better to be a bit too formal rather than a bit too casual, with anything.
"You have to remember there are two ways of saying 'you" in French: the plural and more formal 'vous' and the singular, more casual 'tu'. Never use the 'tu' form with someone you don't know very well- it's a big faux pas and very disrespectful. It's easy to get mixed up, so I always play it safe and stick with 'vous,'even with people I've know for 20 years."
-Jennifer Carter, president of Hermès Canada