I nearly always use a 90 prism on my Rolleiflex. Not only does it give you options to how you hold the camera.. taking lens down or taking lens to the side or taking lens up, and besides orienting the image correctly, it also makes the image much easier to see. You don't have to compete with ambient light and reflections on the surface of the screen as well as you don't have to be taking your reading glasses off and back on. I go the other way from most people on the balance issue. I find it much easier to hold the camera steady with the prism on. After focusing and when ready to push the shutter I cup the top of the prism in my left hand while holding the camera in my right hand. It is very solid.
Dennis