I would say that you´ll be fine as backs have few moving parts. As stated before, your biggest concern will be the fan. My friend still lets his assistants fire up and use his old 13megapixel imacon back for their personal work and the files are pretty fantastic still...
Oh and about the glass and gravity, even if that could affect the back, it wouldn´t have anything to do with actuations (that´s the question here) but with the actual age of the back. Actuations show that the last photographer shot with the back quite a bit, but in general those who purchase medium format backs treat them with great care.