Fall is always recommended.......really doesn't matter what type of plant. The reasons typically given are because temperatures are cooler, rainfall is more plentiful and roots are in active growth at that time of year, so establishment is rapid. It is far less of an issue with dealing with winter cold - especially in your area - than it is having a bunch of new plants facing hot and dry conditions so soon after planting.

Perennials tend to be more seasonal in their availability so may not be as plentiful in fall, depending on the plant. But shrubs and trees should be ready to roll. The only exception I can think of off the top of my head are warm season ornamental grasses. These tend to have roots that enter in dormancy quite early - no active growth then like with trees and shrubs - and so establishment for these is more reliable if planted in spring.

Most (60% of the annual total) elongation of existing roots occurs in fall after the top stops growing - fall in this context is when the winter stem buds have been set and are generating hormones that prompt the root tips to elongate. So on a drought stressed Vine Maple in a Seattle summer this could be happening in July or August.

In addition to having the correct specific timing for each specimen being planted taking advantage of the annual fall root growth push depends on not having to correct deformed roots at planting. This root deformity issue is something that may be nearly impossible to avoid with all but dormant bare-rooted stock being planted at end of winter. Because deformed roots are nearly universal among container grown woody plants.

As far as plant growth cycles etc. are concerned spring is actually a lousy time to plant winter hardy woody plants. However since new roots will grow from cut root ends in spring as long as the industry keeps routinely producing and dispersing finished stock with deformed roots spring may remain the preferred time. The selection at local (walk in) outlets at least is also the biggest by far during that season.

Without question ... Fall.

OK, thanks all, that's what I was originally thinking, but then I
started seeing references to people ("professionals") saying things
like, "most of the failures I see are from people who planted in the
fall." So I kinda starting doubting. But I suspect such failures are from things like people planting way too late in fall, or thinking they don't have to water over the winter because well, "it's not
hot". So I'll go ahead and try to get stuff put in this fall. Gardengal48, glad you mentioned ornamental grasses, they are also a possibility for us, so will definitely hold off until spring for those.