Existing loan and mortgage application

We are looking to apply for a mortgage in the near future. My partner has a 5 year loan and the final payment date for this is dec 2013. We have looked at the nationwide mortgage calculator and it says it discounts loan payments if they are ending in the next 6 months (so we were thinking of holding off till June). My question is is this 6 month rule standard across the board when applying to mortgage companies? Does any company have a longer time frame so we can apply sooner?

Thanks