What you call yourself is the VERY first place you are training clients how to treat you.
What you call yourself absolutely matters in shaping in client perceptions and expectations in the way YOU want them to be set.
If you continually have clients who treat you like like an employee and do not approach the relationship with the professionally respectful demeanor of a business equal, the first place you can begin changing that story is by not calling yourself an assistant.
Because when you are in business, you are NOT an assistant, no matter what the clueless out there try to tell you otherwise.