Don’t forget-

The offer is only 100% complete after the background check which apparently can take around 4 weeks.

Depends company by company - most will give you a deadline for signing (although this can often by extended if you have a compelling enough reason).

If you are sure you want to sign, then no reason to delay. I would only delay if you are also waiting for the outcome of other interviews.

And as the Anonymous commenter says, the contract will generally have a clause stipulating the employment terms are contingent on passing a background check - so technically it is not fully official until this date.