In my outfit it is not actively encouraged, however the local culture dictates that it is offensive to refuse a gift so one must tread carefully. Generally we politely decline, however if the person is insistent that we must accept then the decision is made by the purser as to whether it is appropriate or not.
Generally, the cases I see most is when a single person wants to give the collective crew a gift. It has happened on 3 separate occasions on my flights and has always been a gift such a perfume, a pen etc... (in 2 of the cases it was the same passenger on separate flights, who is very humble and always appreciative of the service- a first class person in every sense who is unfailingly polite and treats everyone equally whether it be VIP or crew member)
Money is also not unheard of although this usually comes from VVIP passengers or royalty. In which case saying no is out of the question! You have to understand that while some people may find it 'tacky' or 'cheap', gifts of this nature are normal in some parts of the world as a thank you or appreciation for good service. Refusal may be seen as extremely offensive.