I agree it is a preference of whether to candle or not.
Blue and dark brown are especially hard to candle. I personally prefer to do those at day 3 to 5 as it is easier to see the singular embryo dot with map lines of veins. By day 10, with those dark or blue eggs, it is really, really hard to distinguish from shell and embryo growth. By 14, you can sometimes see the growing air sac.
I am not sophisticated in my candling though. I use a cheap LED flashlight with a cupped hand standing outside the nest. Gently lift, scoop, candle, return. Hen will be grumpy, but shouldn't protest much as you will be doing this in the dark of night. I haven't tried it yet, but you can place a washer on the face of the flashlight to focus the beam better for brighter light into the egg.
I tend to let my blue and dark eggs run the course unless I make effort to really try to candle with better equipment.
On the other hand, if you really want chicks, candeling can tell you if they were fertile, and candeling early at day 3 to 5 gives you ample time to reset with new eggs if a majority are a bust (You would need to reset all eggs otherwise you'd have a staggered hatch unless you supplement with an incubator).
LofMc