Most of my "assists" get checked early if I'm suspicious (because I've learnt the "too late" lesson) and if the calf is presenting properly, or even if I've had to bring it up/round/into the right position, if there's no reason to pull it (like a backwards one), I let her out into a small yard and let her get on with it herself. It's always a hard pull if the cervix hasn't had time to dilate properly and I'd rather give a bit of extra help when she's down and pushing, than pull in the race, if I can. Most of my cows are quiet enough to get in behind and help when they're lying, calving.
Occasionally we've had to get one back in, especially in the monster-calf year, when we had to pull five of 40 because they were too big.