I took I-5 for a little over 30 miles on southern Washington one time when I couldn't find any alternative on my atlas. Even on 4th of July I found it do-able. The grades are really nice (hills become long rollers, nothing over 6% IIRC**), the two big worries are getting a flat from road debris (mostly disintegrating retread truck tires that shed small wires) and any bridge crossings. The shoulder on interstates is often (almost always?) more than a car width, except for when it becomes a bridge, where the shoulder usually disappears entirely! Taking the lane on I-5 to cross a river in 4th of July traffic was a little hairy to say the least, but I'm here to type about it. If possible, try to time your bridge crossings at anything other than rush hour and preferably early in the morning. Riding alongside a highway, the noise isn't pleasant though, I highly recommend bringing earplugs. I always have some with me buried in my camping gear somewhere when I'm touring, and I always forget to dig them out for the highway sections and regret it! Sometimes the noise doesn't seem too bad at first, but at some point I realize that it's driving me nuts!

Some people don't mind riding alongside highways though. It's the big difference in my girlfriends and I's touring styles. We've discovered that she would much prefer to ride alongside a noisy road or even highway than ride over gravel or hills. I don't mind gravel at all (she HATES it for some reason) and I'd prefer moderate hills and nice scenery to riding alongside the noise and view of a highway. I'm hoping that she'll get to the point where she starts to enjoy the journey more instead of largely seeing it as how you get to where you are going. Until she decides to take on some of the route planning, I'm mostly winning right now though!

If you do go inland, I don't know which route they take, but I believe the annual Seattle to Portland bike ride (STP) takes an inland route and I'm pretty sure their old routes are up on the web somewhere. If I had thought of that before I took my trip from Seattle to Mount St Helens, I probably could have avoided that 32 mile stretch of I-5 I took, but I wasn't on the 'net at all the early 2000's when I did it. Be aware that if you are doing the reverse of the direction they go, there may be a spot here or there that their directions don't work for you, but it should be very minor.

Oh! The other thing to look out for when using highways is the on/off ramps. This is easily the most dangerous part about using highways as a cyclist. Pick a technique*, and then sprint across when it is clear(ish)! Having a mirror is super helpful for this!

* Either cross right where the ramp meets the highway, essentially taking a straight line from your path on the shoulder, or ride towards the other end (where the ramps meet the surface street) to deal with traffic when it is moving much slower. The latter method is safer if there is a shoulder on the on and offramps, but entails a detour each time you cross one, sometimes with a bit of climbing.

** I'm not sure this is true going from California to Oregon, I'd be concerned about the grades on the inland route through here.