I used to be a surveyor. There's no way in the world that any small bubble level device is sensitive enough to allow you to level a camera with respect to the local horizon to any greater tolerance than about plus or minus 5 degrees. They just aren't that precise. The really precise bubble levels, like the ones used on high order survey instruments, are so sensitive that if you shine a flashlight on one end (at night of course), you can see the bubble start to move as the glass vial is heated differentially by the light. Those are good to under a degree (or maybe a lot more, depending on the instrument). Also, in that kind of work, rather than just center the bubble, we read it's actual location against numbered marks on the vial, at several different pointing positions.
Knowing that, if we're interested in absolute alignment, we need to align the image to the local level plane. Levelling the tripod or the camera body is insufficient. If there's an alignment problem between the image (either the digital sensor or the film frame window) and the viewfinder, then you've got to either get the misalignment corrected, or in the case of a digital camera, use live view to match the image itself with the horizon. Thus Jonathan's suggestion. You don't have to shoot with live view, just use it to verify your setup, and then shoot using whatever other method you prefer. Of course, that won't do you much good for handheld work, but if you're trying to shoot architecture or something of that nature handheld, you've got all kinds of other problems.
When I'm shooting panoramas for QuicktimeVR, I use an ancient Canon point and shoot, since all the extra data from a DSLR would be more storage trouble than I need for my low-res result. That particular camera has about a 5 degree sensor/camera level discrepancy (I've heard anectdotal evidence that many or most of the same model are similar; probably an engineering/packaging design decision). I use a bubble level to set the tripod. Then I turn on the live view and rotate the camera around the vertical axis to check alignment. I use the tripod head to adjust the horizontal angle of the camera until I'm satisfied that the images will be level. Then I shoot. Piece of cake, and neatly sidesteps all the issues associated with tripod and camera leveling.