It's always a good idea to candle eggs before setting them. Sometimes you can have hair line cracks that can't be seen w/o the light of candling that could pose problems with bacteria getting in and ooze leaking out. It will help you look at things such as porosity of egg shell which could help determine cause of deaths later on. If you are getting your eggs from an outside source it may even alert you to an egg that has been unknowingly incubated by a broody or bad storage in too warm conditions. (Rare, but a possibility.) Plus it gives you an opportunity to check for air cells and gives you an idea of how fresh of eggs your supplier is sending/giving you.