Situations may dictate issues. You might find a high volume highway that suddenly loses its shoulder, a state highway connecting two expressways, things like that. Or times of day - quitting time around industrial sites, and the previously mentioned prime drunk driving times.
And oddly enough, states with a lot of paved shoulder mileage can have issues on roads where there are none. It's almost like the drivers don't know what to do, and the thought of actually slowing down or timing their passage just doesn't occur to them. On the other hand, I've seen states with no paved shoulders that treat cyclists well, I'll cite Massachusetts here, and most of the Northern Tier states west of the Great Lakes. Highways with very narrow areas right of the fog line are dangerous, some motorists expect you to stay over there even if they're passing with less than a foot of clearance - learn the best lane positioning practice. (The Pennsylvania Bike Drivers Manual is excellent, see chapter 2).
Have a great trip!