I've ridden from WA to MN (and back) at least once a year. We visit Missoula, MT a lot and I've done Lolo Pass numerous times. meh. Its a fun road, but way too many RV's for my taste. If you don't mind a few miles of hard packed dirt, this is a MUCH better road. Dirt from St Regis to the ID/MT border, then paved.
And since I really don't like the slab, here's an alternative from Devils Tower. You can incorporate Lolo into it, just head north once you get to Clarkston, WA. If you insist on heading to Seattle, I highly recommend North Cascades Hwy (SR20). On this route, you even get a free ferry ride across the Columbia River. lol.
There are soooo many roads out here.
Other interesting places to see. Anaconda, MT. Its between Butte and Missoula. Head up Hwy 1 off I-90, stay the night there, have a great steak dinner. Then continue on Hwy 1 back up to 90. OR, from 1, take 38 to Hamilton. Then north on 93 to Lolo and up Hwy 12. You can avoid Missoula altogether that way.
Sounds like a fun trip. We're headed east this year...but it isn't going to be until December. If only there wasn't snow. hahahaha