If u stand high or low on the stairs I think is not nearly as important as standing more to left or right.
Me personally I stand slightly ( half a character model to be exactly) to the left - left when u look upwards the stairs. You can also check my latest vod of stream where I did a 3 hour WoC farm 
" Yalla_Braaa " on Twitch
Edit:the reason why is :
Most mobs come from the left part of the stairs so I focus more on not letting these slip ( where the" half char model"- step to the left comes to full use)
When u stand exact middle u will slip mobs on both left and right side. By choosing one side u will slip no or at least significantly less on the side where u are a little more on.
So right side as mobs and left side has more mobs.
That's why I focus on the left part to have more mobs to spike at the end. 
But overall to have no mobs slipped at all is really really rare ( more rare then a q9 bladed to drop xD)
But I end up quite often with only maybe 4 mobs beeing slipped 