Reservations needed in late March 2018?

Weather in Normandy is more like that of the British isles than of central & southern France, so in March you can pretty much count on chilly temperatures and rain. When you say "modify plans on the fly" do you mean linger a bit longer in one place than another? Or do you mean abandon Normandy and head for, say, the Riviera?

Decades ago, booking ahead was a gamble unless you personally knew someone who had stayed in a given hotel. Nowadays it is so easy to see online what a hotel looks like, and to read helpful reviews, that booking ahead makes really good sense. Some bookings allow you to cancel without penalty up to 48 or even 24 hours before the appointed date. If you wing it, you could end up staying in a really unpleasant place that was forsaken by travelers who planned ahead, or paying a lot more than you want because the only room available is a super deluxe one.

No matter what time of year, I think there's always a risk to booking your hotel late if you are on a strict budget, have unusual needs (triple or quad rooms), or are picky. Wanting to stay in high-end places mentioned in a specific guide book (which is read by many other travelers, most of whom probably book months ahead of time) definitely qualifies as "picky". How disappointed will you be if you have to find a place on booking.com?

I like to maintain flexibility as I travel, but my standards are rather basic. Even so, I sometimes run into a problem and have to pay more than I wanted to in order to find a well-located room. There's always some risk. I usually try to arrange hotels at least two days ahead of time, and I'm trying to stretch that to three or four days.

I would never, ever in this day of the Internet arrive in town and start looking for that night's lodging. Having a car gives you a lot more options than I have, but I think day-of bookings are very risky even if you don't want to stay in the best places recommended by Rick Steves.

I'd book refundable rooms, for the cities you mentioned.
Tourism will be up significantly this year...

If there are 2 of you, and you share a room, booking 1 day ahead of time is an option. We traveled in Bretagne in May of this year. We are cheap, and like cheap hotels - we stayed in hostels (private rooms), business hotel (Ibis), and such. We did not pay more than 100€ in any place. We would stay in a place, and look 1-2 days in advance for the next locale. As I say, this was May 2017. So, in March, I would think that you would have less difficulty. I would wing it, unless you have your itinerary stapled solid. You do have a lot of stops. Have you considered a base with day trips rather than 4 different stops?

If you are flexible, then no problem in March. It's not even shoulder season then. But if your vacation would be derailed if you did not get the perfect room in that hotel you read about last summer, better reserve it now.

Personally, I like to have reservations. With reservations, I don't have to spend time looking for a place to stay, and I have a set goal for the day (I tend to drive too long otherwise). In March it will get dark early, so it will be hard to find a good place. Unlike the US where there is a selection of chain hotel/motels off freeway ramps and in most towns, in France the hotels are smaller and less obvious and vacancy signs are not displayed. If you do decide to "go by the seat of your pants," I would advise that once you decide where you will be going, to find a hotel online and call them in the morning to make a reservation for that night.

I agree with Carolyn and others that you'd be wise to line up your rooms at least a day or two ahead, so you don't lose time searching at your destinations. Or make refundable reservations well ahead (especially for places appearing in guidebooks) -- sometimes they cost a little more but the flexibility is worth it.

I also agree that you've planned a lot of driving and moving around, with not much time in each place. You could limit yourself to one night at MSM -- preferably right on the Mont -- to give yourself a little more time for, say, the Loire. If there's one place that's likely to fill up if you don't reserve well ahead, it's that MSM hotel.

Great Info much appreciated....

I'm the opposite. You will always find a room. You may not find THE room you would have chosen. The biggest obstacle won't be hotels being full, it will be hotels being closed for the season. Booking ahead won't change that. If flexibility matters to you, don't let this site talk you out of it.

I book sometimes and don't book sometimes. I've been just as disappointed with rooms I booked in advance as rooms I booked at the last minute - and conversely just as pleased with rooms I booked last minute. I think it's good to book ahead when your itinerary is fixed, the night before a flight or train reservation, as an example.

Booking last minute doesn't have to be hard. I used to make a list of places I liked then call ahead the morning before arriving. I almost always ended up with my first choice. Now you can use TripAdvisor to find a room for tonight while on the go.

It's also good to book ahead if you're at a major festival. The only time I seriously thought I would be homeless was on opening day of Oktoberfest in Munich. I called my list with no luck, went to the TI for help but it was a zoo, went back and started calling listings in Rough Guide or Let's Go and found a very nice pension at a decent rate (50 marks back then).