Typically no. As long as the septic is working properly, and passes a septic inspection should the buyer request one.
If the septic is not working or fails an inspection it may need to be repaired or replaced.
Because of the water table in St. Croix county it is increasingly difficult to get a traditional septic system, with a drain field approved. A mound system is becoming the accepted system in Wisconsin. That being said, I believe there are at least four legally acceptable types of septic systems allowed in Wisconsin. You would want to consult a local septic specialist before making any decisions on the kind of septic system to install.
I hope this helps. If you have more questions feel free to publish them here on Trulia, or give me a call directly. I wish you the best of luck in selling your home.