Do I need to reboot the system after an upgrade from engine 5400to 5600 using ePO product update task? I looked in the docs and kb, but could not find anything official from McAfee. It will be great if I missed this information to be added in a doc or kb.
It won't be necessary to reboot. Not for an engine update.
You'd consider it if you were replacing drivers (*.sys) files, such as in a hotfix or patch or version upgrade - but even then we don't force you to do it; it's just an excellent idea and we know you should be doing it.