We had no problem finding places to eat dinner at 6:00.
Although I personally like a late dinner, often we stayed at B&Bs that included a huge breakfast spread. We just weren't hungry for lunch but super hungry by 6:00.
Agreed, Rome is very cosmopolitan and the restaurants are in the business of making money. They cater to tourist needs and if they can get three main sittings per day, they make big money. There are lots of restaurants along the sidewalks and you just look through the menus without being committed to paying anything. In busy areas, they are serving meals all day long.
Some snack bars will charge extra to sit down but almost always, you are served at the table and the extra cost is for the service. If you are not sure, just ask.
I was worried about ordering food in Italy. I found it super easy to understand the system. The only problems we encountered were in small villages off the beaten track when the owner did not speak English. Sign language and a few smiles always worked great.
Edited: 6:04 pm, November 22, 2012