PlaidCloud expressions and filters provide use of most non-administrative PostGIS methods. PostGIS methods are accessed by prefixing the standard method name with
ST_SnapToGrid(geometry geomA, float originX, float originY, float sizeX, float sizeY);
func.ST_SnapToGrid(geometry geomA, float originX, float originY, float sizeX, float sizeY)
PostGIS Official Documentation for this method can be found here.
Additional capabilities and usage examples can be found in the PostGIS documentation