I am reading the e-book version of Assassin at the moment as I don't have my paper copy, so I can't quote page numbers and what not. But on the subject of whether or not an assassin would reveal his mark to someone, I have this to say.
In the House of Cernus, right at the beginning of the book, Cernus asks Kuurus (Tarl) for the name of the Mark. I rather have the feeling that he 'would' have told the slaver if he knew himself who it was that he was hunting.
"May I ask," enquired Cernus, "for whom you wear on your forehead the mark of the black dagger?"
I would speak of these things, to some extent, with Cernus, for it was important, though perilous, that he should understand what puported to be my mission. It was now time that certain things should be revealed, that they might leak into the street of Ar"
As we don't have much else to go on, I'm gonna say it's ok to reveal your Mark, so long as it suits your purpose to reveal it.
'Englund is my thirst langwedge and I try to speek it like wot you do but aym knot very gud at it. Fank Ewe'