Tipping a concierge is one of the most difficult task. There are concierges and concierges. If one greets you by name, then you really got one. During your stay you might need his advice for a load of different tasks, cabs, directions etc. For all this don't tip until you leave and you are O.K. with 10 or 20 Euros, depending on the number of small infos you got from him.

Don't give anything for restaurant reservations until you come back. If it was exactly what you wanted and come back very satisfied, then give him a good tip.

For complicated things like an almost impossible plane reservation or hard to get tickets or the hiring of a jet in the middle of the night, tip accordingly high because in order to satisfy your needs he might have to use all the connections he has painfully acquired over years of work.

This is valid for Europe. In the USA the concierge is just a title but they rarely are up to notch except at places like the Pierre in NYC. Don't tip for theater tickets in NYC. The so called hard to get tickets are usually bought by professionals just for the purpose of resell at a big premium. Concierges make a thriving business out of it.