Actually, tipping or giving gifts to the concierge staff is not required. You're already paying for the services of a pre-planner and on site assistance. If you feel that a CM in the concierge department has gone that extra step for you, then by all means, you can definitely show your appreciation. Obviously, one of the best things you can do is to make sure that his or her supervisor is made aware of their exceptional service. Beyond that, it is entirely at your discretion.
I've been experiencing probably the best pre-planning service that I've ever received from a concierge department. I sent a letter to guest services recognizing them and I did send along a group gift for the entire staff to share.
Tammi