Absolutely a big NAY!!!
If a window cleaner came knocking on my door asking me to commit to a contract I would genuinely feel that he was trying to trap me. It sounds so impersonal and unfriendly.
I have a very professional image and a very good relationship with my customers. I try to be flexible and as helpful as possible. None of them ever take the michael and they appreciate my service, but occasionally I have had a customer asking me to stop for the winter. I always say 'Certainly, I will cancel the service and IF i have a vancany in the spring I will come over and requote for you' - Because I have such a good reputation and they know how busy I am they always say 'oh, just keep cleaning them'
Adie