In my experience all of the trains have panoramic cars, but not all of cars are panoramic. Just take a quick look as the train pulls into the station and head toward one of the panoramic cars.
What is nice about the switchback in Meiringen is that the right hand side (best views so far) is now the left hand side (still the best view). No need to switch sides of the train. Move to face the other direction if you like to travel facing forward, but remain on the same side of the train.
Just a thought about reservations, most here see forgoing reservations on panoramic trains as a good way to save money, and I myself have never reserved a train seat, but I am not a tourist and can travel these routes repeatedly. The cost of reserving a seat is likely a very minimal part of your entire travel budget, and if it gives you peace of mind, or can help make your travel experience nicer (if the train is very busy or if all of the good seats are taken), then I think the minor outlay would have been worth it.
There are so many ways to save money, this is just one option and it doesn't make you a bad traveler if you choose to reserve seats. 😄