There are several valid approaches, but the one I usually prefer is this: Not to treat dialogs as places (activities) at all.
Reasoning: A place means, that you can reach it via bookmarks/browser history. Let's say I'm on the customer list, and I click "edit customer", a dialog opens. Do I want to "go back" to the list when I click the browser back button? And will the dialog open again when I click the browser forward button? I doubt it, and believe that a user wants to use the browser buttons to go back/forward entire 'pages' within the app (i.e. a concept that feels to the user like a page), but not open/close dialogs within the page.