They are really easy to test with proper equipment, but since most don't have it I suppose replacing front sensors in a pair is a good procedure.
The 300e will have 3 sensors. The rear one is in the differential and the rear brakes are controlled as a pair. The diff sensor hardly ever goes bad.
if you really wish to beat the system you might try just the right front and see how it goes. It is so rare to find a left front snesor bad that it is a shop event when one is detected.
Bosch Master, ASE Master, L1
33 years MB technician