If you have to go that far to protect your proofs then you have the wrong client.
We all deal with clients of varying degrees of honesty, loyalty, transparency, ethics and most of all knowledge about intellectual property rights and related issues. I certainly don't expect every potential client of ours to be well versed on these issues or to be completely honest about their intentions for coming to us in the first place. That doesn't mean we immediately distrust them and create that kind of adversarial dynamic, but we're aware that nothing should be counted on until the deposit is in the bank and signatures are on the proposal.
We just spent a considerable amount of time measuring, quoting and creating proposals for one of the largest alternative energy install contractors in our area after they sought us out after being completely dissatisfied with the work of our nearest competitor. After inspecting the more than a dozen vehicles already done, it was clear that they needed a new vendor. This is a project that calls for roughly $3,500 in interior wall graphics and four full vehicle wraps to be done immediately then a complete peel and redo on a fleet of another dozen Transport size vans next year. From the day they received our quotes, they kept reiterating how fair the pricing was and all they cared about was being in good hands. They never said a word about shopping much further or that we weren't the front runner to get at least the contract to fill the immediate needs.
After receiving multiple calls and emails about our calendar and when we could start on the first items, a few follow up emails of ours went unanswered and then after having completely friendly and trust building assurances throughout the process, my last follow up was answered with a single, plainly worded sentence explaining that they had gone elsewhere. Boom! It can happen that easily.
Thankfully we hadn't done any artwork or pre-production planning besides the quotes.