We are not in the PNW but we grow all of our trees without watering in the sand plains of Minnesota. Almost pure sand, zone 4a, on the windy prairies. Our secret is this - we plant our trees in the fall. If we order bare root
or seedlings, we hold them in containers until early fall - September - then they go in the ground. Our falls are moist. The trees put down roots and then go dormant for the long, cold winter. In the spring, the snow melt and early rains get them started. We have had a few trees come out in spring with tons of new growth only to be surprised by our long dry summers. They "learn" pretty quick though and despite a wet spring do not flush with fast growth the second year. They grow slower but can withstand any weather conditions.
Here are the fruit plants we have growing this way:
Keiffer Pear (this one is amazingly hardy and drought tolerant!)
Wild Black Cherry
Currants (white, pink and red)
Gooseberry (these are planted in our shadier areas)
Red and black raspberries
We are currently growing out seedling Antonovka apples and they will be put out on the prairie without watering as well. Being in the PNW (and I am guessing with a heavier soil?) you will probably have fantastic results by allowing your trees to fend for themselves. We have good results, but our trees grow slowly. Then again - we are in almost pure sand. That is tough for nutrients and water. We do provide mulch
to our trees and do not remove leaf litter. We get fruit set on our trees and they are productive when they reach full size.
Our farm is currently 100% off grid. We have solar energy
and we bring water for cleaning/drinking. Eventually, we will install a well and then we will set up a drip system in a small concentrated orchard area with more plums, cherries, apples, and berries. The trees mentioned above will still be fending for themselves - we just have too many acres to water everything.
Long story short - yes it can be done, yes the trees will be "stronger" for it, but you will sacrifice speed of growth and possibly fruit set. There is probably a happy medium of allowing them to tough it out the first few years and put down adequate roots, but then water periodically during the toughest dry spells and end up with the best of both worlds.