Save the private keys somewhere. Maybe print them out to have a backup in case you lose them.

Generally you should use the posting key when logging in. If someone manages to steal your posting key, there's not as much damage they can do with it. The others should be kept and only used when needed. Here's a post with an analogy for how they work:

https://steemit.com/steem/@good-karma/steem-private-keys-analogy-2017525t935606z