In my stock photography course my teacher told us to leave as much space as possible due to the fact that buyer want this space to write text on if they need to do so. He told us to think like a potential buyer and not so much as the artist photographer thus a big difference on how you crop and use the thirds rule.
Buyers are not dumb. They can add all the white space they need. If you were thinking like a buyer, you'd not want to spend extra money on empty pixels. It has nothing to do with the rule of thirds or anything else.
