An occasional missed period is normal. However, not having a regular period can increase your risk for endometrial cancer. During a normal menstrual cycle, the endometrium is exposed to hormones, like estrogen, which cause the lining to proliferate and thicken. When ovulation does not occur, which is typical in PCOS, the lining is not shed and is exposed to much higher amounts of estrogen causing the endometrium to grow much thicker then normal. This is what increases the chance of cancer cells beginning to grow. If you do not get regular periods, keep a calendar documenting when and how often you do get your period so you can speak with your doctor.
Keep in mind that this does not apply if you are taking a birth control pill, especially one designed to prevent you from getting your period more then once every few months. The pill keeps your hormone levels low, and endometrial lining thin, dramatically reducing your risk of endometrial cancer.