Do you think it will still be quite hot? Should I postpone the trip by one month. I hate to cancel plans but I don't know if the heat will be miserable. I know the weather is unpredictable. But I thought at the beginning if September it would start to cool off a bit.
This Helpline has a FAQ sticky that links to the weatherunderground.com website for historical date upon which you may make a guesstimate. If you prefer to see the same (probably the same) data in graphical format, as do I, you might try weather.com which lets you make a graph comparing your destination to your home, for any given month. All of the above reflect historical data. Actual weather in the future may vary.
I think it would be a great time!
It should be fine. I have traveled during that time and the weather was pleasant.
Our first trip to Italy in 2009 we traveled almost at that exact time and, while it was warm in Rome, it was never really hot. Being from the Pacific Northwest, and being heat intolerant, I was concerned that it was going to be uncomfortable - and it wasn't at all. You will have a great time - just wear layers!
It depends on your tolerance for heat. But whether you go in September or October, be sure to get rooms with AC. You are more likely to get very hot days in October than rainy ones.