UNLESS you're using a Kodak, Revere, or Wollensak stereo camera, something is missing: a viewfinder level.  If you've experimented with adding your own, you're aware that while a level placed near the viewfinder field aperture can be seen clearly enough, most available spirit levels are big and klunky.

StereoType use to sell a cute little .218" x .938" glass spirit level with a normal 30 to 50 minute per 1/10 inch sensitivity.  (A sample's bubble traveled approximately its width at 1/8" per foot change of tilt).

This level sat nicely on the quad lens escutcheon of the Nimslo camera, centered just below the view aperture.  It worked well as an add-on or replacement level in other camera brands and models, depending on your ingenuity.

For accessory placements on cameras, I usually used light dabs of a "colloid" glue like Duco Household Cement (tm).  It seemed to hold reliably, yet would flake off fairly easily upon removal.  Securing the level in place with a contoured application of black silicon rubber glue might be more rugged and better looking.  Don't use heat glues or the newer "cyanoacrylate ester" glues (the vapor eats plastic optics).  Be careful/sparing with any glue near plastic optics (viewfinder) and allow the camera to sit in the open air (on its back) over night.

The general method is to level a surface (work table), place the camera on the level surface, and secure the level (with its bubble apparently centered) to the camera --placing it under, over, and/or within the view through the viewfinder.  For Nimslo, ensure that the bubble appears neatly centered in the lower gap of the viewfinder's projected frame --while the camera is on the level surface.

We've had no problem installing or keeping a level on our camera in very casual use.

Bigga Note: One of our Nimslos has a projected frame that's quite out of plumb and I bet it's not the only one.  If you've properly installed the level, trust it.
 

(revised: June 30th, 2017)