Up to you if you think you need the extra "zoom". Also depends on where in the USA you are going to, what you plan on shooting, etc etc. I would use very different lenses for a trip to new York than to Colorado for example.
After the trip, u will end up with a duplication at the 18-50mm end. Why not get a decent dedicated telephoto 70-200 instead? It may be a bit more hassle carrying two lenses and changing lenses though, but at least IQ is more decent than a all-round travel lens like the 18-270.
Thanks for the valuable replies. I will be visiting quite a few national parks in the US. I will be visiting New York too. I was thinking about the 70-200 too. Have to weigh the price, the hassle of carrying etc thanks!!!
I wouldn't advise getting a particular lens just for the trip, it may end up being a white elephant after the the trip. If you really must, then get something that will be useful to you even when the trip is over.