It's really bad for them. But what do you do when they start to smell? Mine don't smell, but they're pretty worn from the washes. The last wash put some holes in my knees. But hey, at least they're the naturally occurring holes instead of the stupid factory ripped ones. I only washed them that time because a dug cuddled up against me. I don't care for pets.
I'm receiving a new pair on Monday, which I might never wash.