I have no data or even personal experience yet to say if it's worth it, but I recently used a product called
Woodlife Coppercoat. Home Depot carries it online but not in my local stores so I bought it at a True Value. I believe I first saw a similar product on This Old House, where one of the guys said that cut ends of PT lumber which will be in contact with the ground should get a coat of the stuff. The rationale was that the PT chemicals only penetrate so far into the lumber, and if you cut the lumber you're exposing unprotected, or less protected wood.
Again, I don't know if it's necessary or even effective, but it was cheap, so I figured I would use it.