I‘ve never had to pay a deposit, but i do understand the importance of it in the states for 'disciplining' newbies / unknown guys.
But if i saw a domme several times before and suddenly, without any reason she asked me for a deposit - i would stop seeing her. It's not my fault if other guys let her down.
For me, the most important thing in a long term realtion is mutual trust. As i do care a lot for my mistress and never ever have let her down, this would be a deal breaker.