If you fill your water troughs on day 18, you should not have to open it again until the hatch is over. I never found a need to open it until the chicks are fluffy and ready to be moved to the brooder. Good luck with your hatches.
It will not hurt to open the incubator. Some people make way too much of this and scare inexperienced people into being paranoid about it. The onlly thing it does when you open the bator is you loose a little heat and humidity. The humidity is easily replaced by spraying the eggs with warm water before you close it back up and as far as heat goes it only takes a few minutes for the incubator to heat back up. Folks don't need to worry so much and have a little fun with it.