It is mandatory for many countries and anyway it is strongly recommended.
1) It is mandatory to give an overall value to your move for customs
2) If you make your move (again not mandatory but zero risk does not exist ...) you are obliged to detail a minimum your inventory in case of disaster, we will look at how much you evaluated the package in question for the refund.
We offer our customers a standard inventory but it is not mandatory. It is enough that the document that you make is detailed, encrypted, dated and signed.
The two most used ways to fill an inventory are:
1) Do the estimation of belongings one by one, and add up to know the total amount
2) Give yourself a global amount and dispatcher on the most sensitive items