I've found with assisting that it kinda depends on breed. My lavender orpingtons had a habit of missing pieces of shell while zipping, and I had several die before I realized they do it quite often. So for them, I waited until they got going, then just popped the cap off. No problem, many happy healthy babies.
Seramas also tire out easily and tend to quit without assistance much of the time. So they get help earlier than others. My bantam cochins are bad about slipped tendons, so I stopped helping them hatch.
I only did 2 quail hatches, I can't even remember if I helped any or not. I know I lost a couple after the hatch that failed to thrive. But being so small, I lean toward waiting as long as possible, but eventually helping.
Just my 2 pennies, for what that will get ya! lol