If you had a diabetic child, you would always test before giving insulin to make sure it was safe to give it and to monitor the dose. We believe in doing the same for our cats. Many vets don't recommend it because they think the owner won't do it, however the 2010 American Animal Hospital Association Guidelines do recommend it and several research articles validate the importance of doing so.
The AAHA guidelines also recommend using Lantus (glargine), a long acting insulin. Other 12 hour insulins in cats include PZI, Levemir, and ProZinc. Humulin/Novolin N lasts 6-8 hours, resulting in 4-6 hours with no insulin coverage. Home testing will show that very clearly. As I noted previously, good control may require dosing every 8 hours, to avoid steep rises which happen when the insulin wears off.
Point blank: your vet is out of date and blindly following his instructions may kill your cat.. We live with our diabetic cats; most vets see 1 or 2 during the course of a year. Ask him how many he is treating and how many have gotten off insulin. Yes - cats can get off insulin if you use a good insulin, feed low carb, and follow a protocol for regulating the glucose levels.
Stress at the vet's office will raise the glucose level from 100 to 180 mg/dL, resulting in dose recommendations which may be excessive. Changing the diet to low carbohydrate canned food, such as Friskies pates, may drop the glucose levels as much as 100 mg/dL.