We cut our eggs on day 52 of incubation. I know some people that cut their clutches on day 48 - 50. I have also heard of some people cutting as early as day 45. However, the guy that cut on day 45 also have a high rate of deaths either in the egg or crawling out of the egg. I also know of some people that cut the eggs after the first baby has pipped.
On a side note, alot of the people I talked too said that their eggs hatch between day 57 and 60, our first clutch hatched on day 55. I think this may be attributed to the fact that our incubator has been running on the warm side this year, i.e. 91 degrees. I would reccommend that if you are going to cut, cut on day 52 or later. Lastly, the other option is to candle your eggs around day 40. At that time they typically have enough pigment to see what it is (Spiders will obviously have thin stripes, albinos will have a very bright egg, and no black pattern, pieds will have large patches of white). Some morphs however can not be candled, like Axanthics and Hypos.
Either way you decide, it is all alot of fun.
Thanks,