Hi FMCAD,
I had declined the ITA which I received on 23rd Oct, got the Invite again on 13th Nov.
Ontario PNP is valid for 180 days. As you have got the Ontario PNP i am sure you Express Entry will be valid too till atleast June 2016, so you are good there too.
In nutshell, you are in a safe situation, if in any doubts go ahead and decline the ITA. You will get the ITA again when next draw takes place.
Your 600 points will be there for you, declining the ITA will not have any impact on scores.
Cheers..