While I love using a medium format waist-level finder, I have to admit it seems a bit tough to keep the camera level. The horizon as well as walls, edges etc. tilt a bit easier than when looking through an eye-level finder.
The waist level finder allows you to hold the body of the camera out from your body at all sorts of crazy angles. I have been trying to experiment with shooting it sideways or even straight up.
I have plenty of experience using a 2-way bubble level (like the one in my bio pic), but these kinds of shenanigans called for help with the third axis. I have added a 3-way bubble level to my Hasselblad accessory rail shoe and so far it is proving handy.
No matter which way I point the camera there are always two levels in use, and since the Hasselblad side shoe rail rotates in stepped increments I can even level out angled shots.