The information I got today with my offer letter (my first letter, top choice company, pretty psyched, but not the point) says facial hair has to be "neatly trimmed", no "demeaning or offensive" tattoos are allowed, no visible piercings are allowed, and all tattoos must be covered at customer locations.
It's basically just maintain a clean, professional appearance. You're driving the company's hardware, representing the company, and they don't want you to make them look bad.
