We only ate at casual trattoria's versus any nice ristorantes during our trip this May. I wore shorts to about six dinners in Siena, Florence, Vernazza, & Manarola and never encountered a "problem" with it. (Well, I don't think they spat in our food...) It's not like we were going to be able to disguise the fact that we were tourists. Did I look "uncool" in my shorts to the locals? Probably, but I didn't feel too out of place because the place we went to were probably used serving tourists and were casual enough. It never seemed to hurt our conversations with the waitstaff about our travels, their upcoming travels, etc.