[davidw]

As a born and raised English speaker and fluent Italian speaker, I still don't love the "tu / lei". It's very... binary as to what gets used. Sometimes that's easy, but the marginal cases can be difficult to figure out.

I think the tone and posture when addressing someone convey enough formality. Like if I met Barack Obama, I would be very formal and respectful in my bearing and language, but of course still use 'you'.

[agubelu]

In the weird edge cases you can simply err on the side of politeness and use the formal version. Worst case scenario they'll simply tell you to drop it and use tu instead.