Using Your Own Book For a Class

I received a (somewhat dubious-looking) solicitation to work with a publisher in putting together an Intro to Philosophy reader. This is something I could imagine doing in the future--though I've also toyed with the idea of putting together a 100% open-access philosophy anthology (which would include both classic and contemporary writings, perhaps even some commissioned pieces).

This led me back to the old question: what are the (unwritten) rules, if any, concerning using one's own books in one's classes? (Perhaps some schools actually have written rules about this.) Presumably, if you have created a book for a particular class, it is one that meets your distinctive vision of what content is to be covered in that class. So why not use it? On the other hand, of course, is the sense that one would be making extra profit (on royalties). Of course, someone will get paid (though perhaps royalties aren't that much; I don't know), so why shouldn't it be you? Alternatively, one could defer the royalties from books sold to one's own students, or donate that money to a good cause. (One could even let the students vote.) I had a professor who paid each student a dollar to balance out the fact that one of our required texts was a book by him.

Thoughts?