Maybe it will be more of a guessing game. Depending on the build point you have, you can cover a certain section of the wall. If they happen to choose to sap there, your tunnel collapses their sap tunnel and they lose a portion of their unit. Otherwise, they may be able to hit a section not guarded by your countertunnels and their sap makes it.

This makes sapping a viable though risky strategy against small forces but a bad idea against a well garrisoned city.