
I have never used them, the only time I lost a pup after an long labour with bitch and I exhausted and both fell asleep the pup that was crushed was in the middle of the box under Mum.
I have seen pups get stuck under and especially get heads stuck with pig rails, a bigger risk I feel, so I just try to be vigilant until pups eyes are open, by which time in my breed the danger is past.