Shouldn't do much - diesel purity has come on in leaps and bounds in the last 5 years with the ultra low sulphur fuel grade, can't see your roc getting dirty enough internally in the forseeable to need additive treatment. On the other hand, if your Millers doesn't have a use by date or use within so many months of opening, it can do no harm.
2013 - Tornado Red MK7 Golf GTD on order
2011-2013 - Rising Blue 170GT
2009-2011 - Pewter Roc 140GT
2007-2009 - Tornado Red Golf 170TDI GT
2005-2007 - Black Pearl Golf 140TDI GT
2003-2005 - Black Pearl Polo 1.9TDI