Is it necessary to put some coins in decorating the house?

Generally, you can't put it anywhere. If you know Feng Shui array, it depends on the situation.

In some geomantic arrays arranged by geomantic schools, coins need to be placed in foundations or walls. However, some Feng Shui schools believe that coins belong to gold and have a chilling effect. If not properly placed, the house will be unlucky.

If you don't understand the essence of geomantic omen, don't put it anywhere and don't listen to other people's advice.