booking rooms ahead

You can research hostels/hotels on booking.com to see what's available in your price range/desired location. If there's a festival or special event scheduled, rooms may be harder to get. I've also used booking.com to reserve rooms the day before I arrive.

It all depends on your preferred method of travel. I research lots of options and book all our lodging about six months ahead. The last two weeks of May are the start of prime tourist season in almost all of Italy, so the most popular (and usually the best) places will likely already be booked up. You'll be choosing among the leftovers.

But there is a huge amount of left overs and some of it is prime. The stuff in a Steves' guidebook sells out fast because it is very limited but there are still many, many good options. You will be fine waiting. Careful of a car in Florence because the whole city seems to be one large TLZ - with huge fines. I would dump it before Florence.

If you're carrying a mode of communication (laptop/noteboook), there's no reason you cannot book accommodations on the fly. Many hotels also have computers in the lobby to use for booking rooms for upcoming days. If you're going low budget, at least decide on 2-3 places where you'd like to stay in the above cities before you go. AirBnB.com is a great website for accommodations. And see RonInRome.com for great suggestions on Roman accommodations, and everything to do in this great city. FYI: Your itinerary can be better done by train from city to city. I do like short term car rentals for day trips in Tuscany, going from hill town to hill town (Volterra and San Gimignano).
Siena is best taken as a day trip by bus from Florence. We found it to be very congested for automobiles, and parking very difficult and expensive. The bus leaves from across the street from the train station (by the McDonalds.)

You will always find a room but if you're concerned about staying within your budget I would ahead. It seems like your itinerary is set so why not reserve. There are some EU holidays at the end of May which make it a very popular time to travel.

patricia, For travel in the last two weeks of May, I would highly recommend pre-booking accommodations for the Cinque Terre. At this late date, it may be difficult to find something in the "budget/hostel price range". For the least expensive lodgings, you could try the Ostello 5 Terre in Manarola or the Ostello di Corniglia. Note that the Manarola Hostel has a STRICT lockout in the afternoon (not sure whether the Corniglia property is the same). You could also try Mar-Mar Rooms in Riomaggiore, as I believe they also operate a small Hostel. As you'll be driving, I assume you're aware of the compulsory requirement for I.D.P.'s and the usual warnings about ZTL areas? Happy travels!

I agree with David, try to visit Airbnb. You can save a lot in your penny in Airbnb. Airfares are going up, in spite of the truth that business and personal air travel is also on the rise. The air industry blames federal laws and taxation for the increases, and recently stood up to Congress to say "enough." Meanwhile, economical travelers can find big savings by shopping smart and thinking outside of the box.

I agree that since you know what cities you'll be in, you should book your lodging ahead of time. I personally would not chance it until you get there, but I like the security of knowing that I have a room reserved when I arrive. I don't want to waste precious sightseeing time looking for a hotel with luggage in tow...but that's just me. The end of May is the start of high season so I would reserve now. You probably won't have a great selection since it's already the end of April but I would still hammer down some reservations. Have a wonderful trip.

I travel a lot without reservations and have done so in May. However, given your itinerary (mostly big cities) I would probably just go to booking.com and reserve places where you can. I think that spontaneous travel works best when you are in the countryside, smaller towns and villages rather than coming off the train, bags in tow and having to walk around and find a room... which I have done, but if you are a planner it could be stressful. I have also pre- booked places in some of these cities and then wished I wasn't on such a committed schedule... it's all a trade off.