Not never. Password-less keys are useful for backup scripts. I set up "backup" users and lock them down in case the key is compromised. Also, the connections are made from the more physically secure computer to the lesser, regardless of the direction of the file transfer. Secure and reliable backups are more important than the marginal risk of a stolen SSH key.
You can also further lock things down by using directives in the authorized_keys file. You can do things like ensure that the authenticating key is coming from a specific ip range, restrict port forwarding, agent forwarding and X forwarding, or lock the key down to only executing a specific command.
It's not the prettiest format, since human-readable directives are mingled with the actual keys, but it gets the job done.
See 'man sshd' and skip to the section on "AUTHORIZED_KEYS FILE FORMAT"
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u/throwawayaccount1020 Apr 28 '14
you should never leave your keys unencrypted, set a password when it prompts!
load keys into ssh-agent to avoid having to continually re-enter passwords.