Honestly, I never wash my mountain bikes and seldom wash my road bikes. For mountain bikes, it doesn't matter if the bike looks showroom fresh at the trailhead because it's not going to look like that at the end of the ride. Having a bit of dirt (or a lot!) from a previous ride (or a ride from 10 years ago) isn't going to hurt the bike. It's a badge of honor.

Same with road bikes. I'm not selling a sponsor product so I can ride without caring what the fans think.