Mostly it is a personal preference. I tend to start on the opposite side of the Continent I live on, so that when I'm done, I'm home.
There is a lot of talk about the jet steam and prevailing winds, but that affects jets and birds more than us ground huggers. I have been in Wyoming and the Dakotas several times , traveling both direction, all four actually) and have had headwinds either way.
I've met people going the wrong (other) way and they swear by or (at) the direction their using as being the best or the worst.
I do have a preference when on the East and West coast, as have done trips up and down both. I would not again go from South to North on the west coast of USA or from North to South on the East coast. WInds are a definite factor there. Going north along California and Oregon coast mixes hills and headwinds and you start dreaming of engines!
Self contained touring has the advantage of not having to slug out headwinds. You have your home with you, you are not required to get from point A to point B each day. If the wind is in your face, try a different direction and explore a new highway, wich brings me to planning. Dont carve any itenary in stone(except finding good pie each day), the more flexible you are in that regard, the more fun you will have and the more relaxing your trip!
Where are you planning to travel?