I'm curious if anyone has any feedback on whether it is still legal
to ride the shoulder of the interstate in most of the west.

I'll second the suggestion you've already received about using Adventure
Cycling maps. I rode the southern Tier in a San Jose to Jacksonville
ride in 2001 and mostly followed their route
(http://www.mvermeulen.com/oneyear/usa.htm).

Most of those western interstates are legal for bicycles. However, a
friend of mine who cycled most of the CA-TX distance on interstates had
~8 flats from the small wire beads from disintegrated truck tires. After
he switched to smaller roads, many fewer flats.

I'm open to any hints, feedback, tips, warnings, etc... as I'm a
novice at cross-country touring.

My biggest suggestion would be to make yourself a little less of a novice
by making a 3-5 day shakedown ride or two before you do the cross-country
ride. This gives you a chance to try out equipment and try out your
credit-card light preferences.

My second suggestion would be to assume you might have some days of tough
weather (cold, headwind, rain) and plan with that contingency in mind -
either by having some extra days or equipment or both...

--mev, Mike Vermeulen