I live in North Carolina as we have seasons, it is not a drastic as other parts of the country. I can say this, I have 6 smokers and the only time I have had a problem with mold is in my verticals and that was 1 time several years ago. All of my barrel type or my off set smokers, I have yet to see any mold. When I have finished cookin on any smoker, usually I will clean it out the next day by hosing it out with water but I leave the renderings on the grates until the next cook. I leave them open until they are completely dry on the inside. If your smoker is properly seasoned, you shouldn't have to worry to much about rust. I do on a annual basis preform a "deep clean" on my smokers and after they dry I re season them with a heavy coat of lard and my favorite mix of wood and charcoal for at least 8 hours. That puts the flavor back in the smoker. As far as the grease build up in the bottom, I will usually scrap that out with a putty knife and usually as long as you don't scrub the metal seasoning will remain.