Keep in mind the headlight washers will only work/activate/function when your headlights are on. Some loose guidelines: avoid use at high speeds; if they become stuck in the up position do not force down by hand; if you live in a cold climate be sure you use anti-freeze wiper fluid (search this site for pics of headlight nozzle lines bursting from freezing temps); try not to use them too much in heavy/driving rain; never in sleet & snow & mud splatter (hardcore off road) unless you are certain they have room to go up and down in/out of the bumper and will not allow build-up of snow/ice/sleet/mud/dirt in the nozzle/rail mechanism while in the up position. In these conditions some folks will quickly turn off headlights, spray the windshield-only, then turn lights back on. Probably not the best idea but an option nonetheless. You cannot manually turn off the headlight washer function while the headlights remain on unless you want to fool around with fuses and set-up a secondary switch somehow. If it's a major problem -- driving in a heavy snowstorm at night -- you can pull over, turn off the headlights, spray and then get going again. Probably an edge-case at best. Also, if your Cayenne is covered in snow be sure the headlight spray nozzles are down/in the retracted position before you clean the front of the vehicle. You could easily snap them off or crack them.
I bought a used 2004 S (Certified Pre Owned) and when I first activated the windshield and headlight cleaners (at night, headlights on [of course] going 75/95 mph in the summer, no rain, open freeway) they became stuck in the up position. I noticed them stuck up the next morning before I got into the vehicle. I took to the dealer, they pulled the bumper and lubricated the rails/track. No problem since. Dealer said to active them once in awhile and gave me the above tips. Also told me if I had shoved them back down the CPO coverage may have been a bit of an issue. So far, Sonnen Porsche in Northern California has been great. Also, when you active them them going under 30mph you'll see the spurt of cleaning fluid spray over the light beam. I'm a dork and had someone activate them while the vehicle was in a parking lot idling in neutral so I could observe the headlight operation from outside the vehicle.
Enjoy your Cayenne! We've had ours for just under a year and we still love it. My baby (21 months old now) daughter's car-seat is in the middle of the back seat. Cayenne are heavy gas guzzling tanks and I don't really care -- they are safe as hell and their overall performance and handling defy logic and physics. I keep uncovering cool things -- recently replaced the interior air filer (buy parts from Sunset Imports, link on this site) and while my head was in the passenger footwell I found the two auxiliary power plugs -- I could not believe I never saw them there before -- I read the manual cover-to-cover and saw them depicted and looked (twice on different occasions) but never found them. Very embarrassing I missed them. Then, with my head on the floor changing the filter I saw them. They are tucked way up under the dash. The bad news is now I may have to purchase a radar detector because I have a place to plug it in. I am a sucker. Also, the headlight washer is briefly noted on page 125 of the 2004 Cayenne S and Turbo manual.
PS: Become a contributing member. It's the best money you'll spend on your Cayenne -- you can download manuals and a lot of reference materials. And no, I do not know Loren or any of the Renntech staff -- just my opinion. That said, Loren and folks are truly great guys.