Why would you have a problem saving a family time in line?? It's not like one person standing in line to have a huge group push thier way through the line to join that one person... for fastpasses, it's one for one.
Kari has also told me that she's had people come up to her at Thunder with fastpasses and say that they can't be used. So she then will hand them out to the next family of the same size, saying here... jump the line. When she's working fastpass return, she'll swap fastpasses for people that come early. Like they show up 15 minutes early, she'll give them fastpasses that are good for that time... then 20 minutes later do the same for another family.
The only ride I'd show up in my window for is Toy Story Mania. It seems they only follow the rules to a T on new rides when the wait time is like 3hrs long.
Disney's underlying rule is do what it takes to make the guest happy. And it's not hurting anyone by letting them use thier fastpass beyond the window.
Two major backups happen, 1 when a ride reopens after a down time, 2 after a parade. But I'd just wait about 30 minutes after that when the line starts flowing like normal again.