Here's my take on camera levels, and adding them.
First, the levels are only there to get you close. The real fine adjustment is aligning the image with the grid on your ground glass. If you're doing architectural work and don't have a gridded ground glass, you're bound to get skewed images that you don't want.
The real first step is to make sure you have a gridded ground glass that is mounted correctly (plumb and level) on the camera back. Then, even if the film is skewed a bit, the image will at least be right.
If you don't have a gridded ground glass, you can always just guess (imprecise) or use the edges of the ground glass as reference lines (more precise, but not really great), or get a gridded overlay for your ground glass. I have a field camera that only gets used for landscape and non-architectural work without a grid, and it's fine for that application. My cameras for city work and architecture all have gridded ground glass. I think it's indispensable for that kind of work.
I've mounted bubble levels on my field cameras that don't have them. I like good-quality bulls-eye levels on the top of the back standard (not the removable back, but the camera body that is always in the same position). To mount them, I do the following:
Go armed with a tested and accurate small (6-8") torpedo level, your bubble level (not yet mounted), some double-sided adhesive and/or good-quality double-sided tape, a good pair of scissors and your camera with its nicely-positioned gridded ground glass. Find a nice, new building with lots of right angles and perpendicular lines (windows, doors, sides, etc., etc.). Set up your camera carefully (very carefully, the set-up is the reference) for making an image with all the verticals parallel. It helps to compose the image in portrait (vertically long) orientation.
Set up your camera in "zero" position (no movements applied), level the back as well as you can side-to-side and front-to-back with the torpedo level. Now, get under the dark cloth and spend some time getting the vertical lines exactly parallel with the grid on the ground glass using the front-to-back and side-to-side tilts on your tripod head. Use the pan/tilt head, not any camera movements! These need to stay at the "zero" setting.
When you get those vertical lines all parallel and lined up with your grid (that means both sides and the middle all exactly right), lock everything down so it will not move as you fiddle with your camera.
You will have already chosen (and cleaned) a spot for your bull's-eye level (right?), so now, carefully set the level there and see how it reads. If it reads exactly level, then you're good to go. Get out the adhesive and glue it down. If it doesn't read correctly, you need to spend time finding shims for it and positioning them correctly. Usually it won't take much; a thickness or two of tape strategically placed, etc. Get the shimming worked out and then glue the level, together with shims in the right position. Check again to make sure the image is still correct and that the level reads level.
Note that it doesn't matter how great the level is, since you're basically setting it to read with the bubble centered when the camera is level. That's all your really need.
I didn't like the look of the bare level on my wooden cameras, so I found some brass washers just the right size to fit over the level and glued them in place as a trim ring. Looks great.
For use, set up the camera so that the bubble level reads level (bubble in the bull's eye) and then compose and adjust your shot on the gridded ground glass. The level is just there to get you close anyway, so expect to do some tweaking, especially if you're working with older buildings that aren't always square or plumb all around. Often, you'll need to make an aesthetic choice about what to leave a bit skewed.
Hope this helps,
Doremus
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