From Issue #137September 2005
My desktop at home is running constantly and my SSH identity is registered with ssh-agent. I save the variables SSH_AGENT_PID and SSH_AUTH_SOCK in a file (~/.ssh/env), so that cron jobs can source that file and use ssh happily. Of course it is a drawback the passphrase has to be entered (once) before the cron jobs start running. (In my case this is naturally the first thing to do right after boot-up (2-3 times a year...))
I got excited when i saw the explanation of the -q option of ssh, as that would have been exactly the thing i needed (do not leave log)... But that description belongs to sshd and not ssh. (The article indeed quoted sshd's manpage, but used it in the context of ssh. The similarities/differences of ssh/sshd options are indeed confusing sometimes...) Too bad...