Post-op washes are common practice here, at least in the hospitals I have worked in. It is aimed at bed bound patients rather than ones able to mobilise. They can get to the bathroom and shower or wash as preferred. Here we use Betadine (iodine)
in OR as a skin antibacterial (or antimicrobial, whatever it is!) and it can cover a wide area around the operative site. It becomes quite sticky and uncomfortable. So as far as I have always believed it is for the patient's comfort that we do these washes not for any infection control purpose. Sorry I can't provide you with any policies or anything to back this up...it is just accepted as common practice where I have worked. If any of my patient's refused a post-op I would never insist. It is their decision.
Hope this helped