In my opinion, you ask too much of one caliber to go from prairre dogs to elk! However, if for whatever reason you can only have one caliber, error on the larger ones. As the only rules you would be breaking by using a 7mag to shoot varmits would be of those of economy, you might be breaking ethical rules (maybe the law) by using a 243 on larger game.

Some readers to this post are trying to cover all their bases by trying to get one caliber to be all things just in case they "might" go on their dream hunt to Alaska or Colorado. If the vast majority of your shooting or hunting will be for varmits and eastern whitetail then get a 243. Otherwise I would recommend a second rifle!

Remember, keep to the code "a person can't have too many toys!"