Thanks, Art. I got a 6 gallon carboy today (instead of 5) and the clerk said the extra space would make it so that I wouldn't need a blow-off valve (i.e. that the airlock would suffice).
Yup, that's exactly why my IPA exploded all over my basement and nearly sent my airlock through the floor above, slight exageration, but my 6gal carboy couldn't contain a energetic 5gal fermentation. I now use a blow off tube everytime to make sure I don't have an explosion again.