IF the knife is a very customized design, that you would otherwise have trouble selling, then I would say that asking for a deposit is not out of line.
If it's a general type of knife that you build/sell, then the answer is NO!
Specific circumstances for each situation should be the determining factor.
What you need to think about is that once you have an individual's money, no matter what the amount, you have basically indentured yourself to that individual. In many cases no issues arise, but there is always that customer who will ride you like a rented mule once you have a deposit. It's happened to me several times.....you receive a deposit from a client, then they are calling you every single day, taking HOURS of your time, and always asking the status of their knife. Now don't get me wrong, communicating with you customers is paramount, but I literally have had someone send a deposit, then expect me to talk on the phone EVERY DAY, FOR HOURS ON END, changing everything from design to details as they go, and asking, "how much more would XXX be.....and XXX.....and XXXX. Then, when the knife isn't done when I estimated, they throw a fit. In a couple of cases I have actually sent the deposit back, called them, and canceled the order.
More directly to your situation, I keep a list of individuals who have not followed through on their orders....people who have ordered and then when the knife was completed/ready to ship, declined the knife. In those cases, the individual is required to make full payment in advance, on any future order before I will place the order on my waiting list.
It's sad that things like that have to occur, but in these times, we as knifemakers have no choice but to protect ourselves and our interests.