I have done brake fluid changes every 2 years (with every other battery check). My dealer has usually offered a discount for doing both together. I notice a firmer pedal feel after each fluid change so it is worth the expense to me. Testing the fluid in the master cylinder may not give a true indication because that is the highest point in the system and water goes to the lowest point (generally the calipers). Since there is no "engine braking" from regeneration if the battery is fully charged, the friction brakes could get quite hot during a long downhill drive so moisture in the calipers could boil and cause soft pedal feel or loss of braking effectiveness.