I don't think it is necessary, but I don't think you can power wash after the screening is put up. I just had my deck screened in and the contractor who did it, power washed the deck first, said I couldn't do it later because of drainage.

I love my composite steps because they are not slippery in the winter like wood. But there are necessarily gaps in the composite between the boards so mosquitos and other insects can get in if its used on a porch. So I don't understand the logic of using this for screened porch floors. My dh can attest to the fact that mosquitos go under porches from when he had to do some work under the porch last summer.
Dh put some drainage holes/tubes in our existing upstairs screen porch. He can actually close them off with little pieces of chopstick. Helps when we do spring clean up (with mop like indoors.) He has a plan for downstairs too, but not sure exactly what it is. It seems like you could put some thing in for drainage that could be closed up most the time (a hatch comes to mind),
Kathy