Any major health malady affects the decision-making process of humans. If someone is inebriated, that affects someone's judgment. Likewise, cancer and its related treatments, especially at Ruth Bader Ginsburg's age, will affect her decisions. She should retire while she can, that way President Obama can appoint a replacement with the confirmation of the Senate.
Ruth Bader Gunsburg is one strong woman, she has under went treatment for colon cancer twice and she hasn't missed a thing in the process. Upon reading this question I was expecting to find a different type of cancer that might actually affect the way a person thinks, like a brain tumor. Given that she simply had colon cancer, I don't know why anyone would think this would affect her decisions.
While having cancer is a very unfortunate situation I think that Ruth Ginsburg is doing her job professionally and with a clear head. Being a Supreme Court justice is a prestigious job that only the brightest and most intelligent legal minds ever achieve and I believe these people would almost certainly step down if they felt that they could not make clear and level headed decisions.
There is no reason to believe that because Ruth Ginsburg has cancer, that it should affect her decisions. A leader is a leader, and has that trait to ignore their own woes and worries, and focus on the greater good. The challenge of disease will not make her stray away from her long held past.
Cancer or not, I think that Ruth Bader Ginsburg makes very lucid decisions and is a very bright individual. Just because you have cancer or any other illness, it does not mean that your judgement is being clouded all the time. Of course, I could be wrong here, but don't think so.