Thank you for that whole range of answers.
Here's the scoop of why I ask. I've done this with most of the women I've slept with, all with different results:
One ex, a survivor of childhood sexual abuse, told me to never EVER do that. She had major issues revolving around sex to begin with, but something about not feeling she was in control of her surroundings when she was asleep really freaked her out.
Another ex was ok with it and encouraged that behaviour.
Another one actually did that to me. I was fast asleep and she spooned me and worked me into her. I was in ecstasy when I woke up feeling this hot furnace at the end of my cock. Unfortunately, we had other problems and the relationship was very short lived.
My wife is ok with me spooning and feeling my erect cock pressing against her labia. She's a light sleeper and doesn't want me to penetrate her while she's asleep (she jumps at the very slightest thing that wakes her). If she wakes from her sleep feeling me against her and she wants it, she'd much rather guide me in herself.