Best to tell them what to expect and also a test spot is sometimes a must if you're not sure yourself, as oxidized siding can be very chalky and no hope left for achieving a decent finish. You sometimes cannot remove the dirt without removing the oxidized paint. Sometimes there is no easy way out on these, and what ever you do don't "sugar coat" it to just get the work. If the customer doesn't understand the possible outcome just leave it and move on. Sometimes though, siding can lose a layer during cleaning and still be better than it is now with the dirt,...but this is why a test spot is often necessary if you're not sure what to tell a customer. There are usually one or two sides that are worse than the other, this is where a test spot should be done, and also doesn't hurt to do a test spot on the better sides also.
Jeff