I'm not going to lie. I have. I probably will again.
It's not something I've done often. I usually don't need to, since even when I'm not in the mood for sex, I eventually get there and get off. But, you know, sometimes it's just not going to happen and while we're both normally okay with that, understanding it's normal and whatnot, I also think that, as long as I'm okay with it, it's not hurting anyone if I make him feel a bit better about things.
My opinion is totally different if you're NEVER actually having an orgasm and faking it. That sets you up for letting him (or her) think they're doing a good job, doing it all right, when they may not be. It sets you up to never get any improvement.
But when 99.9% of the time things are perfect, a little fudging .1% of the time isn't hurting anyone.
And hell, sometimes, while the sex is great, I'm just not going to get off and just want to go to sleep! I'd much rather fudge than hurt his feelings or have him feel like he needs to work harder to have it happen, keeping me awake. Of course, I'm also just as likely to say "go ahead and finish, Honey. It feels great but my body is just not co-operating today".