As long as its clear from both sides what is to be done at the stated time, I don't think it's a problem.
Incidentally, I've always known about the "hours grace" at SWCC but thought that after that hour is when rescue is initiated; to hear that might not be the case is a bit worrying in my eyes. Initiating a shout early in case of poor weather etc is all well and good and commendable, but the comment that ""oh, it's just XXXX faffing about taking photos" and they were given a bit longer"..... if I was in need of rescue and someone didn't bother to come to look for me cos they thought I was faffing about, I would be pretty pissed off. If I was nearing call-out, I would not be taking photos. If knew I wanted to take lots of photos, I'd have added more time to the call out sheet. As someone else said earlier - the people who have the best idea of how long a trip is going to take are generally the people doing the actual trip.
Contrary to an earlier statement, I think the "BPC approach" of [call-out time is rescue time] IS the most common, and replies on here seem to reflect that. If anything, I would say a flexible approach is more likely to lend itself to problems - either people pushing close to call-out cos they think they can get away with it, or actually adding less time to the sheet in case people don't come to find them when the allotted time is up! Experience might show otherwise, and I appreciate it's a very different situation when the cave is on your doorstep and you can easily go check out the entrance, etc, but that's just my thoughts. And in think in most cases, the "BPC approach" does work best.