Well, this is a situation where you're own concerns an inhibitions will be the best judge for you. On a sunny day, I'll take my snake outside for a nice sit in the grass, if, and only if I know there hasn't been fertilizer or insecticide used. As for unseen mites, anything is possible, but if it's an area with shorter, mowed grass, then I feel less worried about outdoor pests. I only do this when my snake is already up when it's light out, so I think she enjoys it. That being said, it is of course possible your snake could pick something up it shouldn't