I used to fake a lot when I was younger and in vanilla relationships. I was just too shy and not comfortable enough to communicate what I needed - even in a vanilla way. And then of course I am one of these women who has a hard time cumming in general. God how i envy these girls who don't.
But with my former Master I never have. He once told me that in a D/s relationship, the thing he likes about it most is that it is formed on a much deeper honesty... and I think so too. At least from me to him, I have no idea about the other way but if he asks me if I came I tell him the truth and at the start I always apologised when I didn't cum when he wanted me to - but he made me feel comfortable and its getting better and easier. And no more faking and no more guilt. Yay.
lol of course the whole relaxing my deepest fantasies has a part in that. Admittedly more an emotional part but that's more than I could ask for anyway.