But how about a tall and light person and short and heavy person? The permutations are endless. Not arguing against testing, I think it's great to get more test dummies in the seat, but I don't know where/how one should draw the line.
I think somewhere along the lines of 20, 50, and 80 percentile would work.
Across how many dimensions?
If you're testing those two dimensions, there are four corner cases. It may be that for some reason somewhere in the middle is a worse case, but the extremes should certainly be tested.