My girlfriend also is into biting, and has passed this on to me, as well. We have a similar situation to Lunchbox7... she gets incredibly turned on by having her neck bitten. We definitely leave marks, but we try (not always successfully) to keep them below the neckline of our shirts. My suggestion would be for her to invest in some turtlenecks and you to invest in some shirts with collars, and keep the marks lower on the neck.
An odd side note... a bunch of my friends at school also are into biting, so we tend to actually show off our marks to each other... basically, the worse the bruise, the better. Drawing blood is too far for any of us (at least by biting) but we still get some pretty ugly looking marks on us (my girlfriend and I are currently in the lead
).