Tips will always be appreciated, but they are not mandatory. You can leave cash, or you could actually place a recurring charge on your bill. If you do not tip the cleaners, you will still receive the same level of work each and every time. Cleaners will always offer the highest quality cleaning services, regardless of whether you tip them or not.
That being said, it is customary to provide some form of gratuity. You can offer a tip of up to $20 per visit, and that will certainly be appreciated.