I rarely have a problem with most windows, but if the house does have sash windows, even if the customer is in, make sure you check first whether they are open or not!!
Step back far enough from the house so you can see the bottom sash is all the way down.
If the customer is out you can still do the window, even with the bottom sash slightly open, but you have to be very aware of what you are doing.
Not really recommended though.
Opening lights aren't a problem, if like me you have a pole with the hose on the outside of the pole then you can simply turn the pole upside down and pop the catch on opening lights, or push the window closed.
Again, with an adjustable goose neck, even if the window won't close, you can still clean the opening light, the same with ordinary windows that open, just be careful where your water jet or spray is pointing.
If your customer is in then it is a simple matter to ask them to close all windows until you are done.
Spraying water on the back of a customers neck when they are sat on the loo is not funny....well ok, it is, but you have to keep a really straight when you apologise to them...the giveaway is the shriek when the cold water hits their neck and runs down the small of their back
Ian