Do we need hotel reservations in May

Rod, May is already shoulder season, moving into high season fast in certain locations. This is my take on the "flexibility on travel": it is overrated, and at the end of the day you might having too much headache, too little comfort (or too high expenses) in name of "flexibility". What I usually recommend is that you make a pre-determined itinerary for all your days in Europe, and reserve lodging accordingly, sticking with easy to cancel (up to 48h, or even up to 6PM of the night of arrival) options that will fit you. This way, you can still change plans without financial losses, but know where you have a place to sleep. Bear in mind rail connections between Turkey and the Balkans and the rest of Europe are spotty at best (no trains to Greece are running whatsoever, also no trains between Slovenia in Italy, on top of many cancelled night trains between Bulgaria and Romania and the rest of the continent as from Dec. 2011). You will likely need reservations on some trains as well and, if you do so, it is better to have lodging already arranged.

Rod, what a fantastic time you all will havewhat a great trip you have planned! Some things to keep in mind: if you don't make hotel reservations now, you could spend over an hour on the phone (like we did in Rome) trying to find a hoteland that was for only 1 double. Since that trip in '98 I have always made hotel reservations at least 6 months in advance so that we get the hotels we want so that we can stick to our budget. A few years ago I read on this Wall about someone with a RailPass who was in Bordeaux and couldn't get out because there weren't any seats available for a RailPass reservation! He was stuck there for, I think, at least 3 days. That can sure mess up a person's itinerary! So, before I even buy our plane tickets, I figure out the connections and cost to each city. (ex: this year it was less expensive for our friends to fly into VCE and out of FLR than FCO, so I figured out the best train connections and compared costs....Buying point-to-point was much cheaper for them and for my husband and we didn't need to worry about reservations!)
Hope this helps!

I am definitely a 'no reservations' kind of traveler when I have a car and no set agenda. That way, if I cannot find a room to my liking I can just travel to the next town. But, since you are taking the train and going from big city to big city I would not think it would be fun to start wandering around with your bags looking for a room. It can also take a lot of time trying to find place once you are there, and you are wasting a lot of time. For the type of trip you describe it does not always allow for much flexibility. You might waste more time trying to plan for flexibility than if you just come up with an agenda and try to stick to it. Two couples with no plans and no car could prove quite stressful. Trying to make reservations once you get to Europe will also use up a lot of time if you are calling hotels that are already booked and you have to find two rooms available.
Have you traveled to Europe before?

I normally do no reservations except for my first stop on arrival and my last stop before flying home (because those dates/places are set). I prefer traveling in shoulder season and don't go homeless. May is shoulder season many places but already high season in Rome. We visited Rome in late October/early November. Our friend there didn't even hesitate when I told him we were coming. He emailed back with our reservation information. It was our last stop before flying home, so I probably would have reserved anyway. During that trip we did fine everywhere else without reserving ahead (including Cinque Terra, Verona, Venice, Florence, Orvieto, etc.). Another consideration is how flexible are you on lodging? If you're pretty comfortable anywhere, you don't need reservations as much as if you have a list of minimum requirements to be happy.