Your biggest danger here, would be that the customer has you over a barrel when it comes to changes to the site post live, or during development, because you don't have a healthy deposit, they have less invested in actually completing the work, and if the project never finishes then you don't get paid.
Your change control would need to be written into the contract along with deliverables and time scales on all sides if you were ever to have a chance of getting your money from a customer that decided not to pay.
My guess is you would probably lose more customers mid project than you ever completed, and i certainly wouldnt allow a site to go live without a healthy chunk of the money in the bank.
My personal opinion is that you would be setting yourself up for a serious shafting.
