@verbivore,
I think you are right verbi, but I don't know of an actual rule. My suggestion is to reserve the ampersand for company titles and such, and then only when it's actually a part of the company's own name. Barnes & Noble Booksellers uses the ampersand, so you should too. In one of the old Marx Brothers movies, there was a law firm called Ampersand, Ampersand & Ampersand.
If you treat the ampersand version of the plus (+) sign, you won't often be wrong. If you wouldn't write "Bob + John went somewhere" don't use the ampersand.