You probably are aware that redwood is naturally rot resisitant. Out of all the naturally rot resistant woods, redwood is near the top of the list. There are some wood preservatives out there; Wolman Woodlife for example and I've seen some other products in the big box stores... but, they all will change the color of the wood. The answer to your question is really based on your expectations. If you want the absolute maximum amount of longevity then use a preservative. Redwood can survive anywhere from 10 - 30 years in ground contact, so you have to decide if that is suitable.
See this article on Selecting Preservative Treated Wood for some really good information. If you look closely at the study the recommendation is to treat redwood that comes into ground contact.
Last edited by SecretSquirrel; 07-15-2007 at 08:15 AM.
Reason: Additional info
Thank you for the info. After thinking about it I have decided I am going to apply a clear semi gloss water based urethane to the wood that will be in contact with the ground prior to the install, this will at least afford some protection. Also, noticed yesterday when doing my excavation that the ground is very dry where I am building. It is also on a slight grade with excellent drainage for rain and it has the benefit of roof overhang to keep it dry in everything but blowing rain. After doing the excavation I feel much better about the chances that this wood will last for a while.