Yes. If you don't you will get screwed and have nothing to fall back on. You should have a non-refundable deposit ( its non refundable because this will cover permits, engineering, etc) Second payment should be on first day of work ( make sure this covers your materials, labor and a little profit, and then lastly final payment would should be small. Make sure you contract has in it that a mechanics -lien will be put on the property if payment is not made in full. Have the customer sign the contract. Make sure there is language that states there is no verbal contract and the paper contract is the only contract. If changes need to be made or additional items added during the working period you sign an additionaly contract that is a change order, this will have its owns payments that will need to be dealt with. If you are doing a commerical property or a company and with residential. Make sure both the business , business name dba, and the owner all sign and are held liable both personally as well.  We don't warranty anything that isn't bought from us, regardless if it is the same product. Remember if customer supplies it and you install it they are going to come crying to you and blame you. Why would you deal with any of that when they bought it else where. They didn't pay you to deal with that.