What is more important then a deposit with the contract, would be your progress schedule of payments, another point to remember is what will the amount of purchased material be, consider a job that has special ordered items, these items may require advance payment, in this situation, the contract wording, may allow a larger down payment.
Contract law, is a field of it's own, and every state has different views on contracts, best to get good legal advice up front, it may help prevent problems, that can lead to considerable losses.