var change_search_now = false;

    // Change serach default from selection option to the one with value = '3'
    function changeSearchDefault(select){
      select.children("option:selected").removeAttr("selected");
      var newdefault = select.children("option[value='3']");
      newdefault.attr("selected", "selected");
      newdefault.trigger("change");
    }

And in the jqGrid navGrid initialization:

    onInitializeSearch: function(){
      change_search_now = true;
      changeSearchDefault($('.columns').children("select"));
      $("#fbox_grid_main_reset").click(function(){
        change_search_now = true;
      });
    },
    afterRedraw: function(){
      if (change_search_now){
        changeSearchDefault($('.columns').children("select"));
        change_search_now = false;
      }
      // Add "+" button click handlers to change search column default of newest search row item
      $("input[value='+']").each(function(inputindex){
        $(this).click(function(){
          $("input[value='+']").each(function(clickedindex){
            if (clickedindex == inputindex){
              // Change default
              changeSearchDefault($(this).closest('tr').siblings(":last").children(".columns").children("select"));
            }
          });
        });
      });
    },