I'd say a good way to put it is this: If you're talking about birds, where cylinder choke is most effective (out to 25 yards), 9's are quite effective on smaller and more fragile birds. I've killed ruffed grouse cleanly with 9's, but when hunting only grouse (as opposed to grouse and woodcock, where I usually encounter more of the latter), I'd go with 8's or 7.5's.
You do need to consider the "shot up" thing. I like to use 9's or 8.5's in the 28ga. My 28 of choice has very open chokes (basically skt/skt), and the smaller shot sizes help to fill the pattern. If you're throwing an ounce of 9's with very much choke, you're going to have badly shot up birds inside of 20 yards or so. But it's interesting, reading period literature, to learn that 10's were a pretty popular shot size for woodcock. (Not for me! I've tried them on woodcock with poor results. Birds knocked down but back up and flying, or else a whole lot of very small pellets in them.)
