Okay, I'm not saying that everyone should do this, BUT I got my hatching eggs as a Christmas surprise for my 2 little girls and REALLY REALLY wanted to know if any of them were developing before I told them about it.
Candled on Christmas Eve, which was only day 4, and saw DEFINITE development on 10 outta 12 of them!! All light colored eggs, and I was SUPER gentle about it... they are particularly delicate at this stage... and did NOT discard the 2 that looked "clear" (although they STAYED clear, so discarded around day 10). Looked like little red "spiders" at that stage, just some veins with a darker red "spot" in the middle.
I also candled fairly frequently afterwards so my girls could witness the embryo development, was INCREDIBLE for them (and me!) to see. Mixed reviews on this: some say fine to candle frequently if quick/gentle, others say are risking a decreased hatch rate.
Final numbers for my first hatch attempt: 12 eggs, 2 "clears", 1 arrested development around day 15, and 9 hatched and are doing very well.
I am personally very satisfied with the outcome.
Good luck!!