Reservations Vs. None in Norway - Summer Time

If you are only going to Oslo and Bergen you should be OK...there is a tourist office on the lower level of the train station in Oslo that can help you find something when you arrive. Just make sure there are no conferences, festivals, etc., when you plan to be there...I've had the experience in Europe of finding lodging VERY difficult if I happen to arrive in a city where a big event is going on. I ended up sleeping in the back room of a bar once in Edinburgh...great story but at the time it was not fun! Bergen should also be fine if you're not too picky...it's a popular city though and you may not be able to control quality or location. It isn't a very big place but there are quite a lot of hotels there, even on the outskirts.
Are you planning to take the train form Oslo to Bergen? For that you absolutely need reservations...these trains fill up and you most likely won't be able to get a seat.

I suppose that answers my question! But, our intention was to take the train from Oslo to somewhere midway, spend a night or two, see fjords/nature, train to Bergen, and maybe do a different overnight stop-over on the way back. I'm hoping to avoid the Nutshell and get out a bit on our own, but we are using the trains and not driving.

Would you recommend planning it all out carefully and booking every night in advance?

The only thing about booking reservations on the fly is that it can be time consuming. If you are only in the area for a week you may want to just book everything in advance so that you don't waste any time and you can insure that your lodging will be in a central location and to your liking. For me, although I like the adventure of pulling into a town and just finding any old place to stay, it really takes the stress off knowing exactly where you're going to stay and that you have the basics covered.
Even for taking the train only part way to Bergen, stopping off, and then continuing, you will need reservations. You should get those first since you may not be able to be choosy about dates at this point. Then build your time in Norway around that.

Good news is July in Oslo is "low season". Business travel to Oslo stops the last three week of July. Oslo hotels are cheaper the last three weeks of July in comparison to August.

Bad news is that all the Norwegians are on vacation in the mountains and at the seashore. Having train reservations is a good idea when the Norwegians are traveling to the mountains or return back to town. Look up "Fellesferie" and "Fellesferien".

Yes, I certainly get the benefits of reserving ahead. I guess I always hold out hope that I could be more casual/slow traveling since I won't know for sure how many days I'll want to spend in each place. However I certainly don't want to end up not being able to get a room or to pay $$$$ because I waited to book, so I'll probably end up planning the whole itinerary and reserving ahead. thanks!