I am at day 10 of my hatch and candled a few eggs. As I mentioned in an earlier post, I have doubts about the hatchability of this set. I made some errors pre-incubation and it was very cold collecting eggs. So I chose 3 of the eggs that were collected first and have the least chance of hatching. All three appeared to be fertile. Should I check them all? Do I need to worry about having them out of the incubator long enough to check them all? How easy is it to toss good eggs and/or keep bad eggs?
I candle every single egg, tossing all of the clears and those that quit developing. I do this by day 10. I am not religious about exactly when. Somewhere between 7-10. No later than ten.
The length of time that they are outside of the incubator is not critical, if not excessive. A mother hen leaves her nest to drink, eat, and defecate. You will do no harm. I have lost power, and accidently left my incubator off over night (twice). Each time I still had a reasonable hatch considering.
If you are uncertain about an egg or two, hold on to them. Initially, you will be concerned about tossing good eggs, but after a batch or two, you will gain confidence in what you are seeing. You will do no harm to re-check them after a few days.
Crack open the eggs that you decide are no good. Doing that, you will begin to be able to determine why you lost them. If they are fertile. It does help (it did me) to actually see it.
It is rare for one to rot and explode. If you ever have it happen, you will never allow it again. It can ruin a batch.
There are many charts and pictures online that illustrate the process, and trouble shoot problems. Mississippi State University used to have a very good chart for trouble shooting hatching failures. It was good enough that I printed it to keep as a resource. I am not certain whether or not it is still there.
Ron, west coast Ron, would be able to point you to a wealth of resources concerning incubation and hatching.

Candle close to these days to check air cell development. Eggs can easily stay out for two hours by day ten.
I too remove clears about day 10 in eggs that I can see into. I cannot always see into the Dark egg--Marans, penedesenca and some green eggs. For those I look for increasing darkness in the eggs and use my nose to remove any that stink like sulfur.
Best resource is the Hatching 101 article by Sally Sunshine Remember that the problems in that article are rare.
https://www.backyardchickens.com/a/hatching-eggs-101
I also have a bunch of help and resources in the beginning of the NYD hatch a long thread:
https://www.backyardchickens.com/t/936421/6th-annual-byc-new-years-day-2015-hatch-a-long
This is a trouble shooting pdf: