To auto-update or not, that is the question

Hi All -

Just wanted some opinions on a web app interface I am working on. Right now I have it setup using ajax to update fields on the app upon focus of the browser window. It works great! However, it could prove to be costly in DB calls, API callbacks, etc.

What is the best method to use? The current way, the user does nothing. When they come back to the window, it updates (through ajax, backbone views). Or should I add a refresh button to trigger the ajax?