If you have an indoor /enclosed space,,, you will need an exhaust fan to remove the heated, and/or, stale air,and allow fresh air to be drawn in...if area is larger,,an oscillating fan may also be required to keep air moving in the area... SOG is mainly a space,,many clones at same age/size with identical pots placed side by side in an area,,providing the "Sea of Green" Read a little more about SOG here.... http://www.overgrow.com/faq/browse.php?qa=826
The fan is also needed to keep the plants swaying so they build up stem strength and are able to support a heavy load of bud. Also if you start indoor and finish out, the plant will not be strong enough to last the natural wind and rainstorms