I too use cable ties. It's an easy way to see at a glance if my bag has been opened (the ability to open a bag by poking the zipper with a pencil notwithstanding). I've also opted for shrink wrap on many ex-Africa flights.

If an airport insider wanted to transport illegal material point-to-point, they have better options than checked bags, such as general cargo or even hiding it inside the plane itself. But assuming they did put illegal material in your bag, it's pretty likely they'll have someone at the other end retrieve said material before it even gets to the baggage belt, so you'd never know it happened.

If you are worried about it, ensure there's no address attached to your bag, so they don't know where to go to 'retrieve' said items if the bag makes it to you first.