I decided to reply after all. I went with "Got it. I'll check with you next time."
I talked to my mom and she explained it was just a trim, not like a change of hairstyle. But she's fine with this. And I recalled that XW did check with me before cutting D7s hair once before. So this is fine.
Bottom line, I responded in a way that gave her the information she needed (how we'll move forward with haircuts) without being drawn into the spew.
Me:38 XW:38 T:11 years M:8 years Kids: S14, D11, D7 BD/Move out day: 6/17/14, D final Dec 15