Yes, any wood that will have ground contact must be treated. Other
framing can be untreated if you allow plenty of air circulation.
Don, you were in Southern California long enough to have apparently
forgotten about real weather. As Greene and Greene did in Pasadena,
builders in low-rainfall climes get away with things that simply don't work
in places with weather. And even in dry Pasadena, those exposed rafter ends
have rotted.
Here in Phoenix, there is lots of exposed wood on decks. An extension of my
master bedroom deck and two sets of stairs down to the lower-level pool are
unroofed wood. One stair needs to be replaced, the other stuff needs to be
prepared and painted every couple years. I previewed a house on a huge lot
a few days ago with extensive decks and elevated, painted wood walkways in
the resort-like backyard. I judge the wood to be about fifteen years old.
All the wood has deteriorated to the point where it will have to be
removed, a huge job.
Unless one is building temporary, wood not only needs to be well above the
ground but it needs to be protected from rain and snow, including wind-driven.
An alternative to a wood deck would be a masonry patio or, if the wood look
is desired, a slab covered with Trex or other synthetic decking material.