Here's a translation of a French article about the effect the Amanda Knox case has had on Perugia. According to this, which is all subjective, there's been a negative effect on the economy, notably in the number of tourists and American and English students attending the Foreign school. The Perugians are annoyed and think they've been unjustly punished, however I wonder just how many people quoted in this piece were up at midnight to applaud the verdict?
Thus while tourism for Italy may not be down if one takes economic factors into consideration, this is an indication that perhaps people are choosing other destinations than Perugia. I cannot say I'd be all that enthused about going to Perugia, however I wouldn't hold that against Florence, Milan, Turin or Rome. I would learn a few words of Italian first, an absolute necessity in my mind when dealing with the police in Italy:
'Stai mentendo, voglio un avvocato!'
(Being as I can still kinda read French I'd like to say I translated this for you all, but that would be
really stupid and a complete waste of time as google translate does just fine with French. All I did was the finishing touches)