That is entirely your decision. Here are a few factors to consider:
Do you have a health issue that might force you to cancel your trip? If so, insurance is a good idea but not the insurance through Disney -they have an exclusion for pre-existing medical conditions.
Are you going during hurricane season?
Do you have a grandma or other relative that isn't doing so well?
These are all really good reasons to consider insurance. Mousesavers is recommending
www.insuremytrip.com - they compare rates between many companies and you can get insurance that does not have an exclusion for pre-existing medical conditions.