No issue with overflow or spill. During normal ops, the CTR TK fuel is used first as well. The fuel recirculates to the outer tank and overflows to the inner tank. The inner tank should not be completely full, because of the TO being done with fuel from the inner tanks. I assume that the inner tank will not be completely refilled before the CTR TK is empty.
There is also a vent surge tank in the wings that allows for a 2% fuel expansion.
The likelihood of reaching your destination increases if you have fuel in the CTR TK because the fuel in the INR TK is not needed initially. Gravity feeding can be postponed until the CTR TK is empty and the 30 minutes above FL300 are exceeded.