I agree with Dick's assessment that a full loft should be assumed. In the case of a 24x36 barn, 500 bales are packed almost to the rafters. Although I'm certain someone could get 600 bales in that same size loft, you couldn't get many more.

Another thought is the economy of designing a building using timber framing to carry 300psf. I enjoy timber framing, but that would be one big, heavy and expensive building. Most people would have trouble justifying the expense of a 300psf timber framed structure. I know I would!