Historically, no. I’d guess the workflow would have been author - editor - typesetter - proofreader, with grammar checking being the job of the editor? These days I wonder how many clients would appreciate having their grammar corrected by a designer? Not none? ;-)

“Need” have a solid understanding of grammar? Probably not, but editors and designers both make mistakes. Having a basic idea of what the other guy’s job is about can help keep them to a minimum. Having a CMS on hand is even better. Knowing it intimately is better still.

And that being said, watch out for egos. No one likes to hear that they’ve contributed errors, even if there’s still time to fix them.

It’s pretty helpful for not butchering copy when using it for pull quotes. Or if you write your own ad copy. Aside from that, I’ve grammar is best left to the editors so that they can deal with the authors.