In my experience, it wasn't the last boarding group that forced to check-in roller bags. Gate agents estimate the number of bags to be sent down and tag of those in the beginning of general boarding (group 2, I think). If the bags of very last passengers are sent down, it will delay the departure because ground workers won't have enough time to move the bags.

It won't be the same for different airports or GAs, but bags of E passengers were occasionally safer than group 2 in my limited experience (mostly ABQ).