For me, changing the "Use Kill Shot" option has no effect on DPS, and on the Shot Rotation tab, Usage of Kill Shot box, everything in that box is 0. I have "Boss Fight Length" at 8, "Time spent sub 20%" at 2, and "Use Kill Shot" at "Sub-20%". If there is something else I need to change, it's unclear.
It's not a very big deal because 1 or 2 kill shots is not going to make a signficant difference to PvE dps no matter how much damage they do.
Here's what the Shot Rotation area has:
Usage of Kill Shot
Current Kill Shot frequency 0.00
Possible Kill Shot frequency 0.00
Current Steady Shot Frequency 1.80
New Steady Frequency 0.00
Old Kill Shot DPS 0.00
New Kill Shot DPS 0.00
Old Steady DPS 1400.74
New Steady DPS 0.00
Gain 0.00
% of time spent sub-20% 25.00%
DPS gained using Kill Shot 0.00