You may get differing opinions on this, but the general consensus seems to be that concierge staff won't accept monetary tips but will accept gifts. A lot of people here have sent gift baskets from the WDW florist, some send flowers, some give boxes of Godiva chocolates and some make up little gift baskets with goodies from their hometown.
From what I have heard, the concierge staff are particularly appreciative of these gifts. I think giving the gifts a couple of days in to your trip is a good idea, particularly if you want to single out someone who has been particularly helpful. I think it's up to the individual what to give or when to give it- I'm sure some people don't give anything at all, and I don't think concierge expect it. IMO, it's always nice to get an unexpected surprise, so I imagine concierge staff feel the same!
Have a great vacation