After 5 years doing builders cleans, hand on heart, we have never been stopped retention. Bad cleaning will show up before handover to a client, and not 12 months later, so retention shouldn't be an issue.
If retention was taken out of our money, I would take it up with the QS, and let him sort it.
We have just quoted a site for Redrow Homes, and their T&Cs mention retention, and to apply in writing to be exempted. If we get the site, that is what we will do.