In my experience, using small format, levelling the camera with bubble levels of the kind which are commonly used in photography, i.e. with a small base, there easily is a mistake up to 0.2░ or 0.3░. That might not be noticeable with most subjects, but it is with certain architectural subjects and, in general, it is if you draw rulers on the image.

I think one should first level the camera (or both the tripod and the camera if more than one picture is to be taken with a different angle) and after that, if the subject has a certainly vertical line which can be used as a reference against a ruler, fine-tune the levelling.

Finding a suitable vertical line in the frame is not so easy though. Lamp posts are very unreliable, just like any pole in general. Outer edges of old buildings are also unreliable as they might have some scarp, albeit small. In Paris I saw innumerable XVIII century buildings with a very pronounced scarp and windows following it, so that basically there is no element on the fašade to be relied upon. Water drains are also not necessarily very vertical.

Fabrizio

PS It goes without saying that while using small format the vertical line to be taken as measure is to be on the exact centre of the frame.