I have been thinking about this and I would believe it depends on your job, the relationship you have with your employer and supervisors, and more importantly how much information you are comfortable disclosing. When searching for a job, or maintaining one there are no regulations stating you have to share your medical diagnosis with anyone at your work place. I worked in HR for a long time and it was always an unspoken process when looking for lay offs to choose the people who were less productive or had issues that might keep them from work. This process is a form of discrimination so companies are very careful how they handle situations. The problem with employment today is the 'at will' clause that is in almost every business, corporation, and small establishment. The clause provides the company opportunities to lay off at any time for any reason, hence the 'at will' employee. This clause has released possible discrimination issues with health and/or performance because of health. It also removes the security of longevity, established employees are no longer secure new hires will be the first to receive lay off notices.
Insurance premiums are at an all time high, and every company has a rating, this rating depends on number of employees, age of employees, current employee health, and what diagnosis' each person has. Insurance companies will provide a positive quote when business' shop for health benefits for employees, but the final number is almost always higher after the research is completed by the prospective insurance company; the business pays more and the employee contribution can be higher. One note, if your company is changing insurances or updating, every employee completes a form and you list any preexisting health conditions and medications, this form goes to HR and then is forward to the insurance company for review, so the decision may be taken out of your hands at some point.
Everyone has doctor, dentist, and eye appointments so you are not alone when scheduling appointments during the work day, businesses usually practice some leeway for these absences. If your diagnosis is not affecting your attendance or performance I believe you do not have to disclose anything, unless you feel comfortable doing so. Just keep in the back of your mind at some point you may have to share your information if you have to request an extended leave as Andrew mentioned. I guess I am a bit different, I do not share my diagnosis with many, there are people I speak with often who do not know, and as I continue my job search I do not share my CML diagnosis, to me it would be an unspoken rejection, just because of the possibilities and future performance.
What is your gut feeling about your job, position, work relationship, and how your company would receive the information. I am convinced this is an individual choice, and each person will be looking at different angles. After all the 'stuff' I typed, the one bit of advice I believe is the highest on the list of importance is to not do anything for a time, until you have thought about it. Take all information into consideration but no one should sway your decision from a high ranking HR person, friends, family, or on this site; you know your situation better than any of us. This may sound ominous and that is not my intent, but once you share your condition there is no turning back, until then you have a semblemce of anonymity. My best wishes to you and whatever decision you choose.