We contacted several window cleaners and found that, relative to other kinds of house cleaning services, it is less common to tip window cleaners.
Especially if you have a one-story house, window washing can be a relatively quick and straightforward job.
The Maid to Perfection office that we spoke to said that window cleaners were permitted to accept tips if they are offered, but that "you don't have to."
We don't recommend tipping unless the washers do an exceptional job, or if you feel the job was particularly difficult. If so, you can tip 5-10%, or $5-$10 for a typical 3 bedroom house.