Here is a Google-translated version of what Chris had to say about choosing Montepulciano over Volterra:
I add here that, following a question about why the choice is its fallout on the city that is hosting the filming, Chris Weitz said that he opted for a need for symmetry and photography. It also took the occasion to say that both cities are beautiful. He added that if there is someone to "blame" for this decision then that's him because he was a very strong wish of the square instead of Montepulciano to Volterra. And I tell you my impression when he spoke you could see clearly that he has done a very professional choice based on that of which in his view, the film needed to make up. I have never concealed that among people who appreciate this director, so I am sure that has worked and is working to make a New Moon really the best.If anybody can offer a more accurate translation, I welcome it! :-) Thanks for the tip, Twi_Crazy!
UPDATE: Shire from Twilight Italia, the only fansite at the press conference, says that they will be posting a video of the conference over the next week, and she writes that Chris Weitz was super-nice about answering questions and that Wyck Godfrey, the executive producer of Summit Entertainment, replied that they're really satisfied with New Moon results so far! :-)