I have a pair of Imperials with a vintage finish after washing them weekly for a year. It all depends on what you want. If you want a high contrast fade, then never wash them. If you want a vintage fade, wash them more frequently
I personally don't care enough to do one or another. If they're feeling grody, I'm washing them. That being said, I hand wash, not machine wash.
I'll probably wash my SEXIH07 this weekend, which I'm sure people will be gasping about.