Just be aware that once you start leaving bruises, you will need to let the bruises heal completely before playing rough again. Repeated bruises on bruises cause sub-surface issues, including deadening of the nerves and thickening of the muscle tissue (they will feel like knots and bumps under the skin, and may become permanent).

Both of these effects (knots, and especially the deadened nerves) can lead to other, long term issues.

On the whole bruising might make you both feel special, but they do have health implications. Personally, I don't like marking my girl at all. A nice glow in the dark ass, that tingles for a day, and not a bruise in sight is much more satisfying for me - especially since I can do that day after day and not have to worry that I'm causing deep, long-term damage to her.