It is already possible to specify the default interpolation method to use in WMS requests either at the service level:
or on a layer by layer basis, through the interpolations vendor parameter:
It would be nice to have a way to configure the interpolation method along with the other WMS settings in the Publishing tab of the layer configuration page:
The available options could be the usual Nearest Neighbor, Bilinear and Bicubic, plus some sort of Not specified option that would result in the service default being used. The interpolation method specified in the layer configuration would override the one specified in the WMS service configuration, but would be overriden by a different setting provided in the request via the interpolations vendor parameter.