I don't know what you mean by rainguard. Unless its a name of a specific ice and water membrane. Like grace ice and water shield. If so i would only go 3 feet up. Grace is the best. Its really not necessary in the south because you have no snow or ice. However if you get winddriven rain then absolutely yes. 30 pound felt is fine. However if you want to upgrade to a synthetic underlayment i would say titinium udL 30. Finally the best 30 year shingles in my oppion are certainteed landmark 30 ar or tamko heritage 30 ar. I personally would not allow owens corning put on my roof.
I too, am unfamiliar with any product named Rain Guard, so please clarify what that is first.
Many contractors are using the synthetic felts, but I find 30# felt to work fine and I will let the other roofers finish their experimentation process before I strongly recommend to switch to the new products.