Every country asks in its visa form whether you stayed previously in this or some other Schengen country. Some people stayed in countries which are now in Schengen, but have not been part of Schengen or Schengen has not even existed when you have been there. Should I mention these countries or leave the field blank?
Those visa forms generally don't expect an exhaustive list of your past travels. This means that if you've traveled to today's Schengen countries let's say 50 times in your life, listing them one by one won't really make sense. Yet in case you've entered Schengen twice and another couple of times to current Schengen countries before they joined the union, it would make sense to list all of them. On top of that, you need to mention the year when the journey took place anyway, so they will see that you've been to those countries with a separate visa and not a Schengen visa.
Long story short, be open and honest, and use common sense.
If you know the dates you where there you should probably put them on the application.
Problem is that Schengen was established in 1995 (20 years) ago so since that time you have already changed your passport more then likely a couple of times so even if you mention it you should be prepared to prove that these actually took place especially if these were done prior to SIS, which I think would be prior to 1990.
Long story short what the immigration authorities are trying to establish is that you will go away once the term of your visa or maximum duration of stay has been reached. So if you have 5-6 entries and exits during the Schengen era they may be sufficient but if you feel like adding a list of all of your entries and exits put them on a separate sheet and attach to the application if possible.