Visuals are more important than performance (can you tell I'm not a pvper? )
To a point, of course. I wouldn't push it down below 30-40fps. (I'm getting high 30's in busy areas of Alinor)
So, yeah. It's never occurred to me to turn off grass.
(when I need to turn down stuff, I'll go with the more subtle & GPU-intensive things. Like reflections, AO, shadows)