Kevin, Powder horns can be rounded, they can be flattened. And they can be made in the natural shape of the horn. When the side wall are rather thick, I tended to make them natural shape. When they were thinner, I used them to make rounded horns out of. I did not make too many flattened horns as I really did not care to make flat horns. I was pretty particular about the shape and the size of the horn as well. I did not like to make very large flattened horns, too much of a PITA.
In thinking back, I don't think I made any rounded horns for the first 8 years that I made horns. It took me a while to get into that.
I think it's best to let the horn tell you what kind of horn it wants to be when it is finished. It will be easier and it will come out better.
Of course, as they say, your mileage may vary.
John (Bigsmoke)