Having a Cesarean section should not affect the quality of the placenta. The only thing to be more aware of during a C-section is that there is a greater likelihood of the placenta getting accidentally thrown out, especially if the C-section wasn't scheduled.
If you do have a C-section, be sure your doctor and nurses know you would like to keep your placenta. Usually you will be awake during a C-section so don't feel timid to remind a few times if you feel it necessary. Also, typically you get to have at least one support person in the operating room with you. Make sure your support person keeps an eye on the placenta.
With a little planning and attention, the placenta can usually be kept without too much hassle, even during a C-section, and it is certainly well worth it.