No need to candle eggs untill day 5 .. After Day 5 you will easly see if your eggs are fertile. Than candle a week after that up untill that last crucial 4 or 5 days. The cooling of eggs during candling does not matter. It may even be helpful for duckling embryos . you can learn a whole lot by candling your eggs. And you can remove dead embryos before their eggs explode and make a mess over other eggs in incubator.(