The 'system' context captures networks; much to my dismay, this has been a problem for no fewer than three employers. It's painfully common for things to expect the networks to persist. They don't really consume resources, so I see no reason to invite the systematic heartburn.

When? When there's disk pressure. Maybe some longer term (weekly, monthly?) to keep a lid on things. The image cache provides a benefit, no sense fighting it. At our rate, daily pruning means I might lose hours (through a week) repeatedly pulling the same images.