Road pedals don't like dirt. MTB pedals are made for it. If you are really worried, switch to Crank Bros EggBeaters and be done with it. The SPD-SL cleats, like LOOK cleats and a few others, are made of a nylon composite, and will wear much more quickly if you walk around on them WITHOUT COVERS. Easy answer is to get covers for your cleats and stay with the pedals that you know and like.