All documentation is important, some is more important than other. Certainly your contract documents are critical and must be maintained in both original state and with all review comments. Official changes by date issued must be maintained and noted/referred to in the reviewed contract documents. These change documents should be cross-linked to other documents and project schedule via assigned issues. Contracts must be followed if possible and impossibilities noted.
Internal and external meeting minutes should be maintained by meeting timestamp and annotated to follow-up documentation.
Daily contemporaneous reports (in Field and Office). These reports must be used to maintain schedules and reviewed in on-site crew meetings. (They shall not be modified after completed and signed) These documents are the strongest documents kept of projects. Subcontractors must also maintain and submit their daily reports.
Remember, once time has passed and your memory has faded it is too late to be contemporaneous.
Don't be lazy and put off not keeping your valuablerecords. Not one of us can is capable in returning our visual or verbal brain thoughts to the past with accuracy.
Photographic documentation that is properly identified,adds total clarity of your written documentation. Lose documents left in stacks fail their purpose. Photos properly annotated and carefully linked to your contemporaneous notes are priceless.
Little things that today might seem not important can become the glue that removes the gaps from your documentation.