Think of how air flows, and make sure your venting follows the physics.
Hot air rises, Cold air wants to sink.
Exhausts should be pulling from the top of the grow space, and intakes should be coming in on the bottom.
If you are sealing the grow space to prevent smells, you'll need to ensure negative air pressure. The easiest way to do this is only use fans on the exhaust. It's just easier, and doesn't require "testing" to ensure it's negative.
Besides having fans on your plants, given the type of light you described, you may want to have a fan blowing across the lights/ballasts themselves. Especially if you cannot build your grow room with the ballasts on the outside.
Whenever you are adding multiple fans in a grow room, be it circulating, or exhaust... think of the air path, and just make sure you aren't making the fans compete against each other. (exhaust out, and circulating across is fine... just don't try to do double exhaust, or 2 circulating fans pointing at each other from across the room)
