Do yo have a contract with your house cleaning service?

I have been cleaning houses for 10 years and no one has ever asked for a contract per se but after the 1st cleaning I write up a "service plan" that lists what is to be done and how often(ie-sheets changed every other week , blinds & interior windows done once a month , ect.).I have the customer sign it and go from there.I've never had any issues yet.If you google "how to start a house cleaning business" there are sample contracts.