My motto is the less handling the better, and I have clean hands when I do.
I just love seeing the little heartbeats at four days, the little wigglers at 7 days, and the sloshy swimmers at 14 days. After that, they get harder to see. Recommendations are not to handle the eggs excessively in the first few days while blood vessels are extremely delicate, and not between 11 and 14 days while they go through some critical development. So I generally try to restrict my candling to day 7 and day 18. I rarely succeed.
The other great reason not to candle is the possibility of breaking an egg increases dramatically while handling it. I've never had a good egg break on its own in the incubator.