Like Dave, I used just a poncho-tarp (and a very light, not waterproof bivy) for the first 700 miles, then I switched to a fairly light (single wall) tent at Kennedy Meadows. I'd do exactly the same thing again, maybe without the bivy at the start.
Not much in the way of rain or bugs for the first ~700 miles. At some point in the Sierras you start getting a lot of bugs, typically, and I like an enclosed tent where I can get away from them, spread out my stuff, eat dinner inside, just have a bug-free zone to stay sane in.
Then at some point, indeed perhaps WA state, you could get significant rain. I think a good tarp can handle rain as well as a good light tent, but the tent certainly works too.
So bottom line, my recommendation (or at least "what worked best for me") is a poncho-tarp until the Sierras, then swap for a light enclosed solo tent at the same time you pick up your bear can, use that tent for the rest of the trip.