[henry]

The point of using Omnifocus is to help you to remember to do the things you need to do. Do what ever you need to do to make that happen with the least amount of stress to yourself.

If you need to add an action to OF to remind yourself to reply to an email then do it. If you don't, eg the mail in your inbox is enough to remind you, then don't.

If your reply is dependent on a bunch of extra steps then I'd make a project, but that's just me. It's not costing you anything to have a million projects other than the mental energy to review them.

Do whatever works for you.