Paper filters are impregnated with chemicals to make them resistant to moisture. I've washed mine, seemed fine.
I've started tapping my filter out every time I use the tractor for any amount of time. I also put 3 numbers on the perimeter of the filter and rotate the area of the filter that directly faces the air intake tube. That's the spot that gets the most dust accumulation so I figured it would help to distribute that over the entire surface of the filter.