Well, she's upfront about inconsistencies in Leliana's song. She's quite insistent that she's telling the truth when she originally tells the tale to the Warden
My impression of Leliana's Song is that we are basically playing through the equivalent of one of Varric's exaggerations. The action in the DLC is framed by Leliana's narration, so its a replaying of a story Leliana is telling long after the fact. And the tone & dialogue used by Leliana suggests that she is telling the tale in a similar manner as Varric, and like with Cassandra, to someone who isn't a close friend or lover. She's attempting to tell a good story and regale whoever the audience is. She's playing the role of a bard rather than spelling her guts in an intimate fashion, as she did with the Warden. If I remember correctly, I believe she also admits to embellishing while telling the tale. I believe she says something along the lines of, "Most of the details are true."
Where the are inconsistencies between the Leliana's Song DLC and the story she tells the Warden, I side with the conversation with the Warden being the more truthful of the two. Otherwise, where there are no contradictions, the story in Leliana's Song is accurate.
That's what I've heard on the road anyhow, take it for what it is. 