I don't know if I'd worry about individuals' desks so much -- as a former longtime office-worker, it's been my experience that the users are the main people who maintain their computers, keyboards and so forth. I worked with quite a few people who were religious about sanitizing their keyboards, mice and phones.
Seems to me (a brand-new cleaner with very little experience) that cleaners ought to be targeting public surfaces, furniture arms, door knobs and the like. That's real impractical with the time I have for the offices I clean, but if it would have a significant impact on the spread of germs, I'd make a point of doing it, at least during flu season.
Needless to say, neither of the companies I work for have told me to do anything like that.