traveling without reservation

I would think you would be OK without a reservation if you have one of those smart phones with internet access so you can contact places shortly before showing up so you do not show up somewhere that is already full. I and I think most people make reservations in advance because it may be cheaper than just showing up. I have shown up one time without a reservation but this was in a big popular city in Europe. That was a miserable experience because I had to stay up all night as all the places that I could afford were sold out.

Just remember that the smaller the town, the fewer the accommodation options. In other words, demand may be lower than in a big city, but so is supply. I imagine that in Norway, with its short "peak season," popular places could easily fill up in June - regardless of size.

Also remember that, thanks to the internet, more and more people are booking ahead. A place that, 20 years ago, had 2 rooms reserved and 8 rooms free at the beginning of the day, might now have the reverse - or might be sold out entirely, particularly if it gets good online reviews. If you're driving, you can always push on to some nearly place that may have rooms. But how much of your trip do you want to spend looking for a place to sleep? I agree that if you're not going to make reservations before leaving home, you'll want a device that will allow you internet access, so you can at least book 1-2 nights ahead, and can learn of sell-out situations before you get to a place.

I have no experience in Norway whatsoever, but I've had more trouble finding affordable rooms 3 to 5 days ahead of time in France and England than I did looking 1 to 3 days out in Spain and Italy. So I think you need input from people who have done what uou plan to do in Norway (i.e., not from me)

August is generally busier than June (in the absence of a special event), but I suggest looking on booking.com now for the towns where you think you might stay next year. How many of the places still have rooms? How do you feel about the prices? Not every lodging is listed on booking.com, but many are, and if you're looking for a place to stay at the last minute, it's much more convenient to look at one website than to travel through town on foot, looking for "Vacancy" signs or the local equivalent.

You will have problems finding last-minute accommodation in Bergen, in small fjord towns and tiny places that are very famous such as Geiranger or Voss. Accommodation is already tricky to find in advance, let alone at last moment. You will find more alternatives further north but, then, notice that many people have the same idea of driving up north as well.

Distances between towns and places in the hinterland are not so short at in continental Europe, so there just might not be hotel options just a few km away if everything is full.

We just returned from a driving trip to Norway September 2017. I would not recommend "winging it" for hotel accommodations. Other than Oslo, Bergen, and some larger cities, Norway is VERY rural meaning the hotel can be 30 minutes or over an hour from anything. These small town hotels typically have under 30 rooms so in peak tourist season, I would not risk it because if they are full, it will either change your itinerary or you will be sleeping in the car. Norway does not have interstate highways like we do in the US where you can drive to the next exit. They are local roads so you will not see any major hotel chains except at the airports.

I would book reservations for frontcountry lodges. (Frontcountry lodges that you can drive to). As others note, inventory may be very limited in small villages. You should/could at least book a day ahead to have the peace of mind of know that you will have a space to sleep for the night.

Backcountry lodges (that you walk or ski to) are different. I understand that tradition holds that all travelers are accommodated and no one is turned away for safety reasons. It could mean that extra mattresses are spread on the floor wherever their is room.

No idea. Norway small countryside towns are REALLY small. Even some of what they classify as cities I would call a small town and only have a couple of places to possibly stay so a small town may or may not have 1 option.
Campsites seemed common in certain areas so I think that would be the only safe way of traveling around without reservation.
In July those probably need to be booked in advance as well but suspect in June you could find open campsites.