I think candling is wonderful from an educational standpoint, however I've had four total hatches so far, three under a broody and one in a 'bator. I didn't have a good candler and most of my eggs are dark so early efforts under a broody were fruitless and I relied on the sniff test. If none of them stunk, they all stayed. However a lot of folks like to candle to determine if there are any "quitters" with dead embryos. These are removed in case they start to rot; they may explode and infect other eggs.
When I used an incubator, there were 22 eggs. For fun, I had two "control" eggs that I candled daily for the first 14 days, and every other day up until day 19 after that. I intermittently picked some random eggs to candle, but I didn't want to risk dropping an egg or taking too much time when candling, so only about half were ever candled. None of them stunk. I had 12 out of 22 hatch, and of those seven were either infertile or extremely early quitters. The other three were developed but didn't hatch. Two had pipped internally and passed, and one looked as if it passed a couple of days before time to pip internally. None of them smelled or exploded.
Personally I feel confident that it is safe to not candle, and simply watch for bad smells. I haven't tossed a single quitter in any hatch until mama hen was ready to get off the nest.