I use this method for set default values on beginEditing:
onBeginEdit: function(index, row){
$('.' +$(this).attr('id')+ 'Actions').fadeIn();
var editors = $(this).edatagrid('getEditors', index);
$.each(editors, function() {
if( this.type == 'combobox' ){
var opts = $(this.target).combobox('options');
$(this.target).combobox('setValue', row[ ( opts.defaultValueField != '' ? opts.defaultValueField : this.field ) ]);
}
else if( this.type == 'combotree' ){
var opts = $(this.target).combotree('options');
$(this.target).combotree('setValue', row[ ( opts.defaultValueField != '' ? opts.defaultValueField : this.field ) ]);
}
});
}