Western Washington to Nashville

I was thinking the same thing. By April we have to remove snow tires in WA, so that is a pretty good indication that the passes are all clear with the exception of a freak winter storm. But, since the main highways are all heavily traveled, I wouldn't go way out of the way, with maybe a backup plan if the weather is unusual. For the passes on the west, that means taking I-84 through Portland, when you get over to the Rockies, I'm not as familiar, but I would plan the most direct route and be less worried about the weather on main highways. Now, if you want to go to Yellowstone, or Glacier, then that is different, but I90, I84, ect are heavy truck routes.