Though I've never had a problem transferring pictures from a card to a computer, I've heard DOZENS of horror stories about lost pics from PC users who cut and paste, so I'm grateful that I've never had the issue. My preferred method of transferring pictures is to use iPhoto, which by default leaves the pictures on the card AND copies them into iPhoto. At the END of the process I am offered the option to delete the pictures off the card, which makes a lot more sense to me as I can see that the pictures transferred successfully BEFORE I delete them. When it comes to digital pictures, really you can't be too safe.
I often opt NOT to delete the pictures off the card during the iPhoto import and instead put the card back in my camera and erase the card from there.