Currently you can restrict signing on the G&S page if a C.S.T. has not been selected. Several people have asked that this requirement include users to not be allowed to add any items to the G&S page before selecting a C.S.T.
Example:
Say you have an organization that does accounting by C.S.T.. If a user builds a contract and receives the error when signing that they forgot to add the C.S.T., they go add the C.S.T. and are allowed to sign. However, none of the items added to the G&S page previously will go to the newly selected C.S.T. accounts. The user must clear out the contract at that point and rebuild it before it will go to the correct financial accounts. This leaves a lot of remove for user error since if they forget to clear the contract, it goes to accounting incorrectly anyways.