whos the professional you or them. Do not let your customers push you around. Be confident and sell your methods. Chemical cleaning surpasses hot water except for grease and then there are some great grease eating products that can be used with cold water.
We are TREATING you home and roof. Some areas we rinse to keep the chemicals from drying on windows etc. Most areas we leave the chem behind or if we do rinse to remove webs and build-up we reapply some more chem for preventative. Mrs. Jones, if you called an exterminator and asked them to spray your home for roaches would you ask them to clean up their chems before they left the job? No they are protecting the home now and you wouldn't want to remove them.