TVS protection negative going ESD event
... so that the polarity of the TVS won't matter


Using bidirectional TVS diodes on DC supply lines will not provide any advantage.


  With the bidirectoinal TVS the clamping voltage for negative going ESD event will be higher compared to the unidirectional type.


If you mean that an unidirectional one clamps at, for example, -0.5V and a bidirectional one clamps at, for example, -12V then yes, true. That's why using bidirectional ones on DC supply lines is meaningless (to me). 

For example, let's assume the input is not tolerant to voltages lower than (VSS-0.5V) and you put a bidirectional TVS diode across it. What will happen if, say, -40V is applied to that input?



Generally, bidirectional TVS diodes should be used on AC supply lines, positive signal/power lines with long cables (i.e. GNDs are not equal at both ends), circuits having requirement of common voltages etc. 

And unidirectional TVS diodes should be used on DC supply lines, positive signal/power lines with relatively short cables (i.e. GNDs are equal), logic level signal lines etc.