Yup 3.65 will be the golden firmware if you want cold boot hack. If you want the most up to date and don't mind running a quick app after a reboot then update to 3.68 but you can't downgrade or use enso (cold boot).
If you want coldboot but not 100% game compatibility, stick with 3.65.
If you're fine with taking an extra 15 seconds every boot to run h-encore and leave your Vita in sleep instead of powering it off, then you can update to 3.68 to play some newer games that can't be played properly with MaiDump.