The prospectus clearly states the requirement (though not mandatory)of a sysnopsis alongwith word limit. But the ignou website had their old prospectus and application form from last years up for a long long time so i do not know which prospectus you are referring to- the current 2012 prospectus has stated both.
the application form also required to a put a tick or a cross against the option declaring whether we are submitting the synopsis with our application form or not. However, nowhere does it state that you HAVE to have a sysnopsis in your hand during the interview.
Having said that, common sense prevails as to what will you talk about before a panel? The interview will be more streamlined if one has a topic in hand else the interview too can get difficult like the test perhaps?
9 Copies of synopsis are usually submitted AFTER selection that too in case you are not suggested to make any changes to your current topic/ content.