Ethan, I personally don't like to take any payment up front. I just don't like being in debt to a customer before I have their purchase ready for them. There would normally be a certain amount of trust before a deal was made.
It's acceptable practice, however to require a deposit if the customer wants something so personal, or specific that it makes the knife difficult to sell to another buyer if the original customer backs out of the deal. It may also be reasonable to ask a customer to make a deposit if the knife is to include extremely expensive materials such as precious metals or gems. I would not consider the piece of steel to be used in the knife blade an unusual cost though. I would not do business with a plumber for instance, who required me to provide my own pipe and valves. It wouldn't seem professional to not have on hand at least the basic raw materials of your trade.
I hope this makes sense.
Adam.
The only time ill accept money up front is if the materials are outrageously high. Like high end Damascus or zirco-ti,moku-ti,timascus and its usually only for half the cost of materials. Excellent advice given here