If you're using a quality thermostat, ambient temperature outside the cooler is fairly stable, and don't open the incubator often I think the fan is not totally required. I check my eggs once a week to make sure temps are still right in the egg container (temp gun the eggs directly). I only look for veins 24 hours after laying. Later on, if there is something going on that will impact the veins I'll notice it from outside the egg just the same.
Edit: one thing that's helpful for keeping temps consistent after opening the incubator is to have bottles of water filling the empty space. When you pop it open and it's all air, the air floods out and mixes with outside air and you lose your temps. Water is a very good conductor of heat, so once it heats up it will hold that temp and help maintain stability as you open and close the doors.
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