If you can't see 3 days after the hatch - then yes I'd probably do the warm water thing now that I know about it.
I also have really good LED penlights that at least give me an idea even in the worst marans eggs.
And I've seen late chicks thrive. Sometimes they got laid toward the end of the start and are on a different day or some just do develop at a different rate for some reason.
And hens - have tinyyyyyy little peabrains and often throw out or fail to hatch perfectly good eggs.
I crack most of my failed hatchers because there are things to be learned. But usually now I pip them first and shine a light in - if it wiggles in there I know it's a late hatcher and may require help.
No wiggling and I know it's dead and I can finish cracking it. If you treat them like they ARE alive and just peek, you have a chance of saving one you'd otherwise kill or toss without knowing about it.
Depending on the eggs involved - whether it's high priced eggs(my marans or dark brahmas) or eggs that are from a pen that is part of my project (my partridge rocks or delawares), or just the yard birds, there's a BIG difference is whether I'm willing to lose one I could save.
When I let a yard bird hatch ten eggs I don't even bother the broody til she leaves the nest with the chicks she WANTS to hatch. Often there are leftover eggs - some alive, that I will then hatch.
I also don't mess with them as they teach the little suckers to free range so two broodies with ten each usually works out to 14-16 chicks growing up to sorting age.
This year was my first again (replacement flock) for all my broodies so they ONLY got to hatch yard eggs. This Fall and Spring they'll get to hatch some of my project eggs now that they ARE good at it. God forbid they "learn" on eggs I care about. I shudder to even think about it. Even the first time broody turkeys only got chicken eggs to hatch.
Chickens are chickens - itty bitty brain pan. And even incubating myself when I have candled a LOT during a hatch - you can see some developing more slowly, not absorbing a yolk as quickly toward the end.
I check for life, then toss. That way I don't miss out on a bird I need or want.
But it's always up to the individual, what you want, what you need, whether the egg is high value or not.
