As advice from synopsys, sign-off STA should be four times STA in two corner, slow corner and fast corner.
You should run STA by primetime. Use bc-wc analysis mode.
Set slow library, check setup time and hold time.
Set fast library, check setup time and hold time.
If all above four conditions are timing closured. You can sign-off. That is recommend flow.
But commonly, you can read both slow library and fast library in and check setup time with slow and hold time with fast. That is fine as well