I live in Europe and find it odd to tip Concierges. The few instances in which I did was at the Beverly Wilshire, because we were made feel it would be necessary (which speaks volumes about that dusty old lady of hotel).

As with GMs, there are rare occasions when I do give gifts (to honor incredible stays oder gestures), such as Hermès ties or something of that kind -- given there is a personal relationship that warrants it.