How do you know when to stay and when to go?

Out of fear of jinxing the situation, we've remained mum the past couple weeks but it seems official that we've finally transitioned to sleeping through the night (if you recall the evening of Queen, that was the start of something beautiful). So now, for the most part, the girls are going from 7:30pm to 5am, with a few peeps and whines here and there, but nothing that requires us to get up and rush to feed them or settle them down as they are able to sort things out themselves and go back to sleep. Well last night we hit a little bump that we've not encountered so far in this new era. Genevieve woke up crying at 11:30 - possibly because a friend of ours is in town staying with us and we were still up talking, which is not normally the case - but it was a little more than what she's typically done the past week or two so I buckled and I went in to settle her. Because of my actions Gina will blame the next 45 minutes on me. In my defense, most of the evening leading up to bed time she had acted like her tummy was hurting her - and she's not normally very whinny so I assume something really was bothering her. I don't have a problem letting a crabby kid cry for a bit, but I do have a problem letting her cry if she's genuinely not feeling well. No one likes to be alone in the dark when they are sick. However, after I rescued her, rocked her for a bit and settled her down she decided that she very much liked having the company and was not in the mood for me to leave. So crying became balling became screaming became finger pointing at Daddy. I tell you what, you do a kid a favor and this is how they repay you. So it brings me to the quandary of today's title: How do you know when to go in and rescue and when (and more importantly how long and how hard) to let them cry once you've established this new sleeping pattern? After the 45 minutes or so Gina gave her some water from a bottle and she settled down and went back to sleep - by know the once successful Queen CD was well into it's second time around and I'd tried rocking her and putting her back down 2 more times. So was this really my fault or would she have naturally gone through this sequence with out my involvement? It's a tough call. As I said I have no problem letting them cry when they're just being crabby for no reason, but if she's really in a bad way I want to be there for her, I want to reassure her that when she really does need me I will be there for her. Of course I am also really liking this sleep thing - as is Gina. Trust me. She was pissed at the thought that I've completely destroyed the last two week run of success. I guess this is just one of those things you never really know the right answer to. Like how do you know the mirror in the GAP isn't lying about the way those jeans look? How many jelly beans are in the jar? And why is it that Bjork was ever signed to a recording contract?