Don't you take pride in your work, I really think to make a job look nice the whole window needs cleaning, it's a shortcut just to clean the glass, after all we are called window cleaners, not glass cleaners. What is the point of just cleaning the glass when you could stick your price up a little and include the frames, you wait and see how happy the customers are, and how much their house will stand out compared to the neighbours. Remember the frame is a large percentage of the window and for a general appearance of cleanliness the job should be done properly, unless the frames are flakey. It's fly by nighs that just do the glass and I will get picked over any other window cleaner in the area because of my standard of cleaning.
You might not like the idea of this non frame cleaning people, but the next fresh job you get, clean the frames, even if it's without telling the customer you include the frames, see how nice it looks and see how it makes you feel, it makes you be able to leave a job with a smile on your face, and I don't know why you'r self employed if you don't want to be happy in your trade. You probably started window cleaning because you were unhappy in your old job, then get happier and do a proper job.
Doing a proper job leaves you feeling good inside, trust me