If you are applying on the basis of 5 year then I believe you need to complete 5 years not even a day less. No matter how you apply NCS or By Post. Others will give you same advice too. Home office is very strict in the time period but they have shown some flexibility if you have been out of the country more then 450 days. Just make sure you were also in UK five years prior to the date you are going to apply. If you apply October 1 st 2006 make sure you were in the country on October 1, 2001, else your application will be rejected.
Thats how I understand their rule.