Normally it is commited, otherwise you get an error. You can add a custom commit handler. (right mouse click on action: set Error Handler).
My best practice: You should NEVER return false from a before commit event, because you won't know that the object is not saved*. Either return true or throw an exception (using community commons for example)
* Unless the object still has the ischanged/ isnew flags, you can use community commons to check them.