Incorporation provides no shield whatsoever, at least in the US. They can still sue you into bankruptcy.
What you want is to spell out in the contract that anything you provide is an "instrument of service" and that it cannot be distributed, modified, blah, blah, without your permission.
And yes, you need a contract for each and every single job you do, no matter how small. There's always the chance that you will have an insane client (like I did) who ran up nearly a million dollars in costs arguing with us, and then ended up paying a quarter of that, when we calculated the original remedy would cost $1,800. And yes, the client was insane, absolutely bonkers. And I had bought that job from another company that we purchased, and they did not have a strong enough contract, and the job was tiny, about $4,000.
Being incorporated does absolutely nothing for you to protect you from that.