How long do you let them go without pipping?

[ascottlaw]

Don Soderberg's book suggest that if all the eggs do pip within 48 hours of the first hatching, then the babies in the non-pipping eggs could suffocate. I'm heading into 72+ hours since the first hatching and still have 6 eggs that have not pipped. Two did start pipping today and they look ok so far. So here's the question: How long do you wait before taking action and what do you do? (I'm thinking about cutting them open a little after the ones that are hatching now emerge).