I've never used a fan in any of my incuabtors...I'm using a "mid-sized" chest freezer turned on it's side - Been using it for last 4 or 5 years and haven't had any problems yet...2 foot of 11 in heat tape keeps my temps between 88 and 90...It'll only hold 12 shoeboxes though...I would imagine though if you have a full size frig as an incubator you might want one...
BT