I should note that rigging a whisker pole and rigging a preventer is a bit time consuming and if you are only going downwind for a short time, then typically you can free fly the jib (without a pole) and run wing and wing without rigging a preventer, as long as you pay attention.
I should also note that on most boats, in light to moderate winds, it is faster and safer to stay on a broad reach with your jib filled to leeward rather then run dead downwind.
I would also note, that few whisker poles are rigged to take a pole lift. Unlike a spinnaker pole which can be double ended, since most traditional whisker poles are single ended (meaning a latching hook at the mast and a non-latching pin or hook at the clew of the jib) and the pole is too long to do a dip pole jibe under the jib, jibing a whisker pole that does have a lift is a major pain in the butt and so since whisker poles are generally significantly lighter than spin poles, even on boats with whisker poles rigged to use a lift, a pole lift is generally not used with a whisker pole.
Lastly, since the death roll that can occur, if you accidentally jibe with a preventer set in heavy air, is more dangerous than the accidental jibe itself, preventers need to be easily eased. And since there are huge loads imparted into the boom and mast if the boom tip hits the water with a preventer rigged, preventers need to be resillient to absorb the shock. Offshore racers generally mount the preventer blocks on a resilient mounting. The use of a comparatively light nylon line (one size smaller than your jib sheet) is recommended for cruising boats.