We all do things differently. Honestly, if you are thinking of cutting always wait until day 55 at least...just to be on the safe side. Also from the temperatures you mention you're incubating at. I would say they would probably start piping around day 52. I say this because this year I had somewhat of an issue with my incubator and the temperature inside the tub was reading at 91.5. I left it alone since the eggs looked good. Guess what? All piped on their own on day 52 and came out healthy as can be. No deformity issues or anything.

Here is a good video explaining how the eggs look before they hatch.