Yep, I threaded my computer fan cord through one of the air vents. I made it so I can disconnect it when I open the incubator to candle or re-arrange the eggs (even with a fan you can still get some temperature variation, rotating the eggs can compensate for this possibility). Having it run while you open the incubator can blow hot air and humidity out, making it take a bit longer to recover.
Other than that, I left it running 24/7 until the hatch was finished. I did place the eggs in baskets, to the sides of the incubator not directly under the fan. I have heard of chicks popping up and getting their head whacked by the fan. Another option is to cover the fan with wire "hardware cloth" or cheesecloth. Basically anything that will still allow airflow, but keep little chicks out.
Another bit of advice, run the incubator a few hours with the fan in there. You may find it does things to your temperature, and you may need to adjust. IIRC, the temp in mine went up after adding the fan. My theory was that since the fan was circulating all the heated air, the element didn't need to run as long to evenly heat it. Anyways, I turned the thermostat down, and it worked like a charm.
Here's hoping all goes well with your eggs!
