You can remove iTunesHelper from starting on login by going to System Preferences > Users & Groups > click on your name (e.g. the current user) > Login Items > click on iTunesHelper and press - remove button to get rid of it.
If you check Activity Monitor, I think you will find that itunes helper is only hidden - it still launches and runs in the background. Hiding processes at login merely stops them from popping up splash screens, etc.
It's really only useful if you have other apps set to open at login.
As you see in Activity Monitor, the parent process for itunes helper is launchd.
Stopping itunes helper requires fiddling with launchd. itunes helper is likely associated with other background processes, so if you disable itunes helper, other things may start failing. It may not affect performance much it at all, but may fill various system logs with error messages, etc.
It's been a few OS X versions since I bothered with this (and finally gave up). It never helped, took a lot of time to keep up with, and sometimes broke things.
For those who prefer to learn for themselves (as I was), I recommend starting with Launchcontrol from Soma-zone. And maybe read this:
http://www.launchd.info/Regards,
PK
