I never run the grammar checker at all. In fact, the first thing I do when I install Word is turn off grammar checking. Then I turn off the option to check spelling as I type.
To me they're irritants that get in the way of writing. Every time one of those green spell check lines appears, it stops me in mid-flow and makes me go back to fix the misspelling. Yes, I know I don't have to go back, but if you're not going to stop and fix, then why ask to see the green line in the first place? So I turn it off. That way I actually write story, instead of a lot of correctly spelled words.
The grammar checker on the other hand is actively bad. The grammar checker thinks it can write better than me. It's wrong. The grammar checker has no idea of voice or style. And don't get me started on those style evaluation systems. If you took any great story of the past and ran it through grammar check, do you think it would pass?
When the option to check spelling as I type first appeared, I used it all the time. I noticed a strange thing. My spelling became much worse. Okay, my spelling was never great, but that made the slide all the more noticeable. And I became slower at writing, because I was always conscious of not wanting to provoke a nasty green line.
It probably does help that I use autocorrect all the time. But that works because autocorrect doesn't make me stop and redo.
So what I do now is, I write the story, and then I make the story right.