There is one time you want to avoid being in the Yellowstone or BadLands area: the weeks before, during, and after the Sturgis bike rally. Google for the dates.
The entire region will be full of groups of people going to and coming from the Sturgis area on motorcycles. They particularly like the Beartooth pass.
Unless you like having your outdoor experience accompanied by large groups people competing to see how much ear-splitting noise they can make---then it would be fun.
If you are in the Glacier area those weeks, you will likely miss most of them. It's not on the way to anywhere except relatively unpopulated areas of Canada.
