Regarding Penile Cancer, it is even more rare than male breast cancer. From the American Cancer Society:
Circumcision is the removal of a part or all of the foreskin at birth or later on in life. This practice has been suggested as conferring some protection against cancer of the penis by contributing to improved hygiene. However, the penile cancer risk is low in some uncircumcised populations, and the practice of circumcision is strongly associated with socio-ethnic factors which in turn are associated with lessened risk. The consensus among studies that have taken these other factors into account is that circumcision is not of value in preventing cancer of the penis. It is important that the issue of circumcision not distract the public's attention from avoiding known penile cancer risk factors -- having unprotected sexual relations with multiple partners (increasing the likelihood of human papillomavirus infection) and cigarette smoking.
So there ya go. Nippin' the tip has nothing to do with preventing cancer...it has more to do with other risk factors, INCLUDING not taking care of your dick! Making sure it's clean and free of debris and smegma. Men who develop Phimosis (which is when the foreskin constricts and has a hard time retracting, thus making cleaning difficult) are much less likely to be keeping up with hygiene, therefore causing the condition themselves. This IS a risk factor for penile cancer. However, the fact they are not circumcised matters little...it's the fact that they are dirty and not cleaning themselves regularly and properly. Men who are circumcised and don't wash regularly are also at risk for fungal and bacterial infections in the skin of their penis. It's all about taking care of yourself, not lopping off skin. Women have a smegma-like substance in their nether-regions, too...but no one in the civilized world would suggest cutting off a woman's clitoral hood to prevent infections. It's the same thing.