Should the JavaScript Defer Attribute Ever Be Avoided?

I'm new to web development and am trying to develop good habits from the start. I'm creating a new HTML page and have placed most of the Javascript in external files, which I am then deferring using the defer="defer" attribute. I am using DOM Level 2 event handling to map a function to the load event in one of these files, and would like to be sure the load event would never fire before the deferred Javascript has been processed. My testing has so far been successful, but there is so much I still don't know... (I'm reading "Learning Javascript" by Shelley Powers which has really been a great help.)

Thanks,
Ann