Servos are sealed - to a point. Some are specially sealed for special applications. For the most part, you're ok as long as you're not getting too much gunk on them.

If you are, you can always make some sort of shield or cover for them.

Little toys that come in those "Plastic bubble glued to a piece of cardboard" type packages can be a great way of getting a plastic shield. And it makes you a hero to some 8 yr old when you get him or her an unexpected toy.

"Just let me keep the plastic bubble that it came in!"