If the window looks clean and is clean, there is nothing wrong with skipping it. You're getting paid to keep your customers windows cleaned.
I've skipped some, but I have found that it's never really worth it. You might save 5-10 minutes on the job, but is it worth losing a client or having that thought in the back of your head?
At the very least rag off the dust. Also depends on how often you are there.
Don't get all high and mighty and say it's stealing - oh please. Stealing? Half the time some $12/hr fast food employee comes and "cleans" the window the next day and makes it worse than had you not cleaned it at all.
Do what you think you should, as it's your business. If there are negative consequences, then it's up to you to decide how you want to run your business and if you find that practice acceptable or not.