Grammar signifies more than just a person's ability to remember high school English. I've found that people who make fewer mistakes on a grammar test also make fewer mistakes when they are doing something completely unrelated to writing—like stocking shelves or labeling parts. ... I hire people who care about those details. Applicants who don't think writing is important are likely to think lots of other (important) things also aren't important.
Many practical tips on grammar and usage appear in The Art of On-the-Job Writing.