These rules might be different for different species (or different people.) If I had chickadees, which I don't right now, I wouldn't do open-box checks at all. In my limited experience with chickadees, they are a lot spookier than bluebirds and more easily frightened away from the nest site. Tree swallows, on the other hand, are fearless, and will dive-bomb you while you are doing nest checks. Some people even report the female sitting tight to the nest and not leaving, even after the box is opened. I never worry about scaring the tree swallows away!
Here is my current crop of Eastern Bluebirds:
To answer Susan's question from the last bluebird post, no, the single egg in that photo wasn't a dud. Depending on what time I check the nest during Hatching Day, I might have 1, 2, 3, or even 4 unhatched eggs, waiting their turn. Usually, most eggs hatch in the same 24 hour period, but I have had one egg go a day longer a couple of times, and once I even had an egg hatch 3 days after the rest of the clutch, just as I was thinking about tossing it out. The runt struggled to keep up, and fledged a day later than his buddies, but he made it in the end.