The short answer is that employers cannot ask you about any disabilities, unless it relates to you being able to perform the functions of your job.

As you can see, the answer really varies based upon personal circumstances and the job culture.

I never told my employers about my condition. They would find out after some time. I had employers that supported me for 6 or 7 years. And I had employers that only kept me for 2 or three years. Though they could not legally fire me for my disability they gave reasons ranging from personal incompetence to corporate down-sizing.

I let mine know because I worked around machinery that I could get hurt on or fall from.

I always tell my jobs because if anything happens they can't say that you didn't tell them that you had a illness . its like when you get your license .