How early do I need to book restaurants in Rome?
Having been to Rome 4 or 5 times, and going for a 5th or 6th time in mid October as well, we don't plan on booking until a couple of weeks before we go for the 1 or 2 "musts" on our list. The rest we'll just leave to booking a day or two before, which always seems to work out well for us.
It really does depend where and what night. For instance, Taverna dei Fori Imperiali is quite small, and books up at least 4 days in advance. But most places you'll be ok if you reserve the day before. Flavio Velavevodetto, Perilli and Trattoria Monti all get full on weekends.
Lat year, I made reservation for only one restaurant (Metamorfosi, and it was not even full!!) in advance (2, 3 weeks by email).
All other places, I went ahead and reserved on site for either the same day/evening or for later in my vacations (Pierluigi, Ditirambo, Roscioli ) or walk-in (Bir & Fud, Tonino's)
I would at least check the best way (AND TIME) to make reservation for the restaurants you wish to go to, (telephone, web, email, ... ) and make reservation 2, 3 weeks before you leave.
If you speak italian, it will be much easier, especially if you carry a phone with you to be able to call a restaurant during the day to ask for a table for the evening.
Disclaimer, I was alone, and did not really care if I missed out on the "best".