I am in the same boat as you with COR and RNG mains and some DNC and THF on the side. I went 12 STR and 12 AGI.
1) Only JR and Evisceration benefit from DEX of my most used WS
2) Exenterator, Wildfire, Last Stand (all of which I use most) benefit more from AGI then Evis or JR benefit from DEX
3) I'm Mithra so my DEX is already pretty high

But when SE offers us more merits into that category, 12 DEX is my next plan.