All cleave aside from the double nuke on Korven/other on hc all cleave on Klaxxi is pure padding. So if you go for maximum effective dmg you use SR "normally", for padding/ranks/giggles you just spam hb.
When to use SR "normally" goes by a fairly easy rule for me, SR > hb if 1.only 1 target, 2. no trinket procs up. Pillar and 2 set stacks can't beat SR, only really huge impacts or cleave/AoE do.
Even on 2 targets i don't use SR, SR shouldn't be realistically better, maybe abit, but at latest with pillar+crusader which should've been it for SR, no actual math just guess though.