What's your system when riding with a partner?

My girlfriend and i went for a ride today. I tried coming up with a system to call out traffic to her. (i'm more experienced and not focusing on directly in front of me isn't as much of an issue for.) How we ride is side by side with me close to the middle and her close to the shoulder. I check behind for traffic lots and when i see a car coming i'll yell traffic and let her get in front so we can both hug the shoulder. Do you think that's safe? Suggestions?