
I've never let mine go over by more than 2 days, from the first mating date. But I know mine go into secondary inertia so rather than just let them sit there, I involve my vet. And you are right, if the puppies just sit, they grow bigger by the day making a C.Section more likely to be needed (size). And if there are only a few, they may not be eager to come out either

I found with our litters that they'd all go 'quiet' (less movement) a day or so before whelping - maybe because they are moving into 'position'? The first time this happened I freaked, thinking 'they've all died'!!! Hah.
It is true that conception may not have taken place when she was mated but again, I'd not let her go more than a couple of days over. It doesn't hurt to at least talk to a vet, much as all too many have NEVER actually bred a bitch and they are inclined to adopt the wait attitude, leaving it up to you to insist!