No. You'll pay a bigger premium for a longer term needlessly. It sounds like a salesperson is trying to convince you to put your cover in place now rather than waiting until your mortgage is ready to draw down. Apply for your mortgage protection for the exact amount and term to match your mortgage. Don't start the policy until a week or two before the mortgage is due to draw down.
As advised by Dave Vanian put the cover live about 2 weeks prior to drawdown.
If there are delays then most lenders will allow you a month or two before requiring what is known as a shortfall letter.
A shortfall letter is a document you sign confirming you are aware that the end date of the mortgage protection is a few months shorter than the end date of the actual mortgage & that you acknowledge this shortfall & would still like to proceed regardless.