States are supposed to recognize other states' laws, but they don't always do it. It's called "reciprocacy". It's that way with helmet laws: if you're riding through a helmet law state without a helmet (and you're from a non-helmet law state), that state's law enforcement is not supposed to ticket you.

If it's a legal gray (grey?) area, and they do ticket you, it'll be a hassle for you to fight the ticket (which they already know).

Here's a possibility: make your own plate bracket, which can move between vertical and horizontal! Then, check out the state laws of the other states you'll be riding into, and move the plate accordingly. Or, just ride with it horizontal when you're out of your home state, and go back vertical when you get home.