Julia Huston Nguyen
My approach is similar to Mark's. I work with a lot of academics, so on first contact I use the Ph.D. in my signature to signal that I come from a similar background and understand their questions/concerns. Once the initial contact has been made, though, I revert to my first name because I want our working relationship to be collegial.
Within the agency, we're all on a first name basis, regardless of position.