I include this on my quotes:
“Nominal specks of paint and mineral deposits will be included in the normal window cleaning process, but excessive amounts will either need to be added to the quote or left as is.”
You get to decide what is nominal and what is excessive based on your time and bid for the job.
I NEVER give them a choice! Outside only when the insides really need to be done for example. I give them a quote to do the job the right way. If they don’t want it done properly they can call someone else.
PS
If it’s done the right way they’ll be happy.
Well. Let me see what may happen here…
You charged Mrs. Jones $330 for her house inside out. You come to the house and figured out that there are bunch of bushes that you have to wack, some of the windows have storm windows, bunch of double hang windows, and by the way she wants you to clean tracks and screens too. Is your price still the same? And,
Just did a quote today for a job, exterior only. They are having drywall work done on the inside and want to wait to do the insides later. Fine with me, I’ll clean the exteriors for my exterior price, then in a few months either interiors or both interior/exterior if it has been more than 3 months - most likely dirty by then again, so winning situation for me.
On residential I am charging about 125/hr. I’ll do whatever windows, whichever sides they want, whenever they want. As long as they pay the rate…they get whatever they want done.
Do they usually complain about you doing a crappy job on the outside. Call you up for a redo?
Then you have to go back and prove that the crap is all on the inside. Unpleasant. They accuse you of doing crappy work and they find out they are WRONG! Unpleasant.
Your situation is different with the drywall not being done yet and all.
Sounds like you charge by the hour. Sweet. Hand them a bill when you’re done. 
In my neck of the woods they want the price up front.
No, never. I am sure to point out afterwards that the windows would look so much better if both sides were cleaned, then tell them “Perhaps next time”. There is no opportunity to say “I” did a crappy job if “they” chose to only have the exterior cleaned.
I don’t charge by the hour but do land in my “expected hourly range” that I place for myself - some land over, some spot on, some land under - it all works out in the long run.
If they don’t want to do the inside the windows usually look like crap. They are almost always unhappy. They made a mistake and it’s my fault. I stopped doing one side jobs over three decades ago. Haha
I think you’d have to be there. The complaint usually came in days later. They wouldn’t look bad until the sun hit them the right way.
That’s why there is terms and conditions
100% True.
I once had a customer that did that early in our business; the day before this job I asked “Lord show me what to print in my Description of Service” The very next day we get the worst customer you could imagine and an answer to prayer. We now have a full page of our services described so the customer knows what to expect printed on the back of all our estimates. We also do no itemize our pricing! Inside and outside and outside only pricing.
I really would not worry about it. If you make it clear while there that the windows will only be half clean then if they blame it on you it is absolutely laughable.
They can still be dissatisfied with the finished product and usually are. No problem. 
