I’ve tried it both ways and found that either style works well as long as the females get along with each other. If they don’t get along you’re going to have problems whether you are keeping them together to raise babies or putting them back together after they give birth. I like to holdback 2-3 females at a time, raise them up together, and keep them together for breeding.
If you want to add another female into an established group (because sometimes fertility and health issues happen) it’s best to do it with a freshly cleaned cage and a male rat. The male should keep all the females in check long enough for the girls to get to know each other. Don’t ever put a new female in with females who are pregnant or nursing, as they will attack any rat that smells different to protect their litter, or soon-to-be litter.
As long as the girls get along, I don’t notice any major differences in loss of babies, whether they are separated into birthing tubs or not. But if they are fighting separation may be necessary to save the babies and prevent injury/further injury to the mother. For that reason, I’m currently using maternity tubs for all my rats. Some of them have done well in communal birthing tubs in the past, but some don’t and I don’t want to chance higher losses while I have BP eggs in the incubator. Gonna need all those pinkies and fuzzies.