Cutting the egg or the membrane?

If egg cutting, I agree with Brian: most do it out of curiosity or because they just can't wait to "see what they've got." Personally, I don't cut eggs unless there's a reason for it (potential problem with an egg).

If you're asking about the clear membrane around the baby within the egg, don't cut that, ever. If you cut it and the baby isn't ready to breathe on its own yet, the baby will die (and that would really suck, to incubate all that time and then kill the baby right when it's supposed to be hatching).