The concierge is the exception to the general tipping custom in the US. No need to tip for routine assisstance.

But, when the concierge lands you the tickets / table, etc. you wanted and haven't been able to locate, something decent is appropriate.

This is especially true if it's a property you routinely frequent and where you want a long-term relationship with the concierge so that you can call ahead and have them take care of things for you.