David,
My family and I took a trip to Grand Teton and Yellowstone around that time this year (indeed, we stopped by Estes Park on our way from Denver, as well).
If you've suffered elevation sickness at Estes (~7500 ft), do keep in mind that Yellowstone averages 8,000 feet (but goes down to about 5,000 feet in the north). Apart from that, it *is* a wonderful time of year to go; the crowds are always omnipresent, but less so than even a few weeks before late August; and the weather is usually sublime.
FYI: one of a few blog entries I wrote last year upon our return home (to northern VA):
http://tao-of-digital-photography.blogspot.com/2012/09/the-same-and-not-same.html