The 15s absolutely killed me on my 1st cycle and nearly did the same on my 2nd. And since it was my first and second cycles I went with 1 set throughout all the rep ranges and I still got tremendous results.
For my third I'm shooting for 1 15s, 2 10s, and maybe 3 5s if I can handle it.
I think its more an individual thing. I just turned 35 a couple weeks ago and just don't have the endurance for multiset 15s using HST, but I'm sure I had enough when I was younger. And age is just one factor.
From what I have learned, in my humble opinion, I really don't think multiple sets of 15s are necessary anyway. Your focus is on joints and storage capacity of your muscles/capillaries, not on hypertrophy.