Where is the best place for an O2 bung and wide band O2 sensor?

[Karl Ellwein]

Is there an ideal location in the exhaust to place the wide band O2 sensor?
Is there a less than ideal place?
The easy place it seems would be at the cat-delete, "test pipe".
If the wide band O2 sensor were to be located in the test pipe would it be sensing a tiny bit of air drawn in from slight leaks in the header flange gasket?
The collector area of the headers would be best right?
thanks,
Karl Ellwein