I've contracted many times as a python programmer for companies but I have never considered charging a 'deposit'.
I recently started my own company and I have been asked a few times now to pay 30-50% up front and to me this seems like a horrible practice but it is also my first time hiring a designer.
So for a designer to work with a startup is high risk. Now of course if you're Steve Jobs after Apple and you're starting NeXT then yes Paul Rand will work with you. But that said I don't think Paul Rand would have even spoken to Steve Jobs when Apple was in the garage in the 70s.
PS This isn't just for tech startups, but for any sort of startup.