As far as safety goes, I'm pretty sure you can keep a container in the fridge and add to it throughout the space of a single day, as long as you freeze it all that night. As far as an imbalance, it seems like doing it at the end of feeding would give you mainly hind milk. But I'm no expert. When I have pumped I always nursed ds on one side and pumped from the other- that's the limit of my experience.