Like synssans said, there is no right answer for everyone. Your best choice depends on your situation. If you need to control front-to-back airflow, either in this rack or adjacent ones -- now or later -- you should use a closed rack with blanking panels in all unoccupied spaces.
What are your reasons for considering an open rack?
I personally think that closed rack is what I would go with, but have been in many companies that have open rack so I think it really depends on the cooling in the room.