Hatching IS nerve-wrecking!! I can tell you that on my first hatch, I had a chick pip and zip and cheep away, but didn't break through the membrane. I decided to leave it alone, and it ended up dying in the shell. This is my second hatch and I had a chick pip yesterday. When I got up this morning, it had zipped and another egg was half-zipped. They were both peeping away, but not progressing and the membrane looked dry. No other eggs have pipped yet. I decided to intervene and gently pulled the shell off and opened the membrane to free the chicks. I told myself that I would stop if there was blood or if the yolk hadn't been absorbed. Now that they are free, they are wiggling and peeping and looking tired, but okay. We'll see how they are doing in a little while.
It's a really hard call, and you have to use your own judgement. If the chick has only pipped and is still peeping away, you might want to give it more time to zip.