I've only just started brewing but this is my knowledge on it. As far as I know you don't have to worry about it if you are only doing a primary. I believe if you secondary that is where you want to minimize headspace in the container, because there isn't much C02 being produced anymore.
If you use a bucket, I would make sure to only open the lid once you're ready to bottle... so your not letting that protective layer of C02 escape. I guess taking hydrometer reading might be difficult in this situation though.
Just my insight on this, someone can correct me if I'm wrong