We would like to do the 17 day Italy trip as our next Rick Steves trip but I am concerned about the weather becoming to hot as it gets south of Florence in the Sienna and Rome areas. To go in 2013 the earliest we would be able to leave would be on the 6/21 trip which is now wait-listed or sign up for the 6/23 trip. We would prefer to go in mid May, assuming the weather near Rome to be cooler, but would need to wait until 2014, which wouldn't necessarily be a problem. Would the weather around Rome have become to hot by July 7, 8, and 9 to make the trip uncomfortable and make a mid May 2014 trip a better bet. By going mid to late May the tour is in Rome at the end of the first week of June at the latest.
Was in Florence visiting my family from June 28 to July 12 this year. Was so hot and humid I was ready to die. When the outside temperature in the car thermometer showed 35 C (95 F) was when it was cool. Went to my summer home in the Casentino Valley (40 miles east of Florence) surrounded by mountains and forests. No relief even there, except deep underwater in the pool. There are generally no significant differences in temperatures between Florence, Rome or Siena any time of the year. Climate in Rome is slightly milder since it is closer to the coast, so they might get that Ponentino (westerly) breeze in the evening, but don't count on more than one or two degrees' difference and be ready for scorching hot, you'll feel like jumping into the Trevi fountain together with the coins (just like in La Dolce Vita).
Mid May is much better.
In my recent experience, weather isn't as predictable as one might expect. Earlier this summer there was a cool snap for a couple of weeks; last year at the same time it was blazing hot (mid-June both years). There were also a couple of rainy days in July. Generally, I think if you can get there in May-June you'll be more comfortable, but expect to use a sweater or long-sleeved shirt over a tank top or t-shirt (mornings/evenings can be a little cool in May).