Read the book The Leader who Had no title by Robin Sharma. He philosophizes this concept and applies it to life in general. You don't need a title. If you do, you're not busy working enough.

My business card simply reads Without title under my name.

Think of it this way. When does another person come across your card? When you hand it to them.

If you are serious about building meaningful relationships with clients/vendors/strangers, a title on a business card should not define your role. You should.