I rarely ever paid for a camping spot with any type of utilities. A few times there was E&W but never sewer.
That said, if I ever did have full hook-ups I might have used the water to fill the grey and black tanks on the first day and do a good flushing of any floaters. Then close the valves and use normally throughout the stay and then repeat before leaving the site.
Essentially, full hook-ups are akin to having a dump station at one's site without the hassle of an actual dump station so why not utilize it. It's paid for, why not take advantage of it.