I-90? Boring!
You must be on the clock with this run. There are so many fantastic roads from central Montana to the coast, and how many chances do you get to ride them? Seriously, check Highway 12 (via 93 from Missoula Mt.) into Washington State. 99 miles of twisties, but new, wide pavement, and all over 35MPH (mostly 45MPH sweepers)! It would probably only add a couple of hours to the ride, but well worth it.
I heard it was snowing from central Wyoming up into Montana as late as yesterday, but they are forecasting in the 80F range by next week over there. You should be good to go through that section.
I'm also down in Portland (with ZG and a few others), so definitely let us know when you make it east of the Rockies. You might want to alter your return trip to catch some great roads just a little further south on your way back (i.e. Columbia River Gorge). I'm sure we can find some 'tour guides' around here (actually any excuse to ride will do). The ride up to Mt. St Helens Johnston Ridge Observatory is great (just did that one 2 weeks ago). The road around Mt Rainier should be clear too, and it was fresh pavement last fall.
Just something to think about while you're droning away on the super slab.