The problem with telling them is that if they don't want a diabetic working for them, they'll use any excuse to fire you or lay you off. If you miss work because your BG levels are too low and you go slightly hypo, or if you go slightly hypo at work, they can say that you aren't doing your job or you miss too much time, and let you go.
I'd never tell my employer anything about my health. They don't adhere to the HIPAA regulations, so anything I tell the doctor or nurses at work becomes common knowledge throughout the plant. Heck when they give us our mandatory yearly physicals, they take 5 or 6 of us in at a time, and do the physicals on us in front of each other, like the military does.
We've had employees take the physical where the doctor found the employee had high blood pressure. Once they find that out, they won't allow you to work until you get it under control. If you can't get it under control, you never get to work there again. The last time they forced me to have a physical, I didn't eat anything for 30 hours so my BG levels wouldn't show anything abnormal.(they use the urine strips to test for sugar in your urine.). I wasn't on any meds at the time and I knew what they'd do if they found out I had diabetes, so I fasted for 30 hours and it worked.
If it were me, I wouldn't tell them anything....even if it was illegal to not tell them, I'd break the law and still not tell them. No good can come from it. But that's me....I'm a very private person when it comes to my health. Only my doctor and I know what the state of my health is, and that's how I like it. Telling other people won't help my health, so why tell them my personal business?
Unless you feel like you're jeopardizing your job and you don't want to take that chance, I wouldn't tell them anything.