Should I wash the roof?

[Forrest Williams]

I'm going to be listing my house soon. The exterior has been freshly painted, it has new windows and doors. The roof is in fine condition, it's just a little dirty. I can pay $400 to have it washed. Is that worth the expense or is roof washing a gimmick? I'm also fairly handy, is it something I could do myself with a few hours to kill? The house is about 1800 square feet, so the roof is fairly big. Is it worth the time or money? I want to list it next week or so.