I try to follow this rule in my own home, and I encourage every one of my clients to do the same. It's a surefire way to rid yourself of clutter.
This isn't to say that William Morris is a minimalist--far from it. In fact, he's a genius with lushly detailed decor. He may have done most of his work in the mid to late 1800s, but it can still feel so fresh. And you know Will and I both love a good pattern mix.
Even his working drawings have style.
Seriously, listen to the guy. He won't steer you wrong.