Great question, if you copy the campaign the greeting will also copy over but if you start with a brand new template you would need to re-add this. Please note, some templates have a greeting by default (that you can remove if you want) so you wouldn't need to add it here.
At this time the setting can't be saved in your account but this is a good idea for the future!
Yes, these instructions are for using our Legacy Email templates. If you're in our newer editor, you would want to follow the instructions found here.