I typically candle the whole incubator once a week when I put new eggs in. I set one of 3 trays every week, moving to hatcher for each group.
At day 7, I through out obviously glowing yellow clears if any and remove bloodrings. Anything questionable I leave in. Typically can see nice veining in the good ones.
At day 14, I check and toss any not developing while I'm loading up the day 0s and checking the days 7s. That way when I go to move them to the hatcher, I just move them over at day 18.
Works for me, anyway. Great hatchability this spring thus far.