There is only one place where I have regularly used and tipped a concierge - at the Sheraton Santiago. He has arranged for show tickets (prior to arrival in Chile), other occasional special arrangements, and on this last trip, even purchased a cellphone for me out of his own pocket - all ready to go and activated with plenty of prepaid minutes, in time for my arrival (which I of course reimbursed him for immediately upon arrival). The tactful way to tip is generally to leave a note in an envelope with the appropriate sum.