I think it's whichever you like best, but I would not go more than 1 or 2 shades from your main color. (I know this isn't exactly what you're looking for, but you're in the right range... http://www.the-preserve.org/public/color02.html)

I would go the actual color. Always paint a sample square on the wall first. Paint then in different areas so you see how the colors acts in different lights.

For me, the same shade will not really be an accent color. An accent color would have more contrast - in a different color. But, if you want to keep the same color but in a different shade, I would do either the darkest, or the lightest so you can tell there is a contrast. (Testing on the walls first to see which is best in the spaces you want).