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.

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.

I think somewhere along the lines of 20, 50, and 80 percentile would work.

Across how many dimensions?