Wash N' Wax is just the automobile equivalent of Shampoo & Conditioner for the car. Just wash & go. I use Wash n' Wax just for quickness and convenience but use a proper car polish once a year to protect the paint and never have any issues.
It really depends on how good a job you want to do...and how much time and money you want to spend.
If you want a REALLY good result, take a look at this site:
These products have always confused me as any decent wash product should strip contaminants off the bodywork. My best guess is that the actual 'wash' part is not very powerful.
Having waffled all that, I have found them to give a good finish, so long as the car is not absolutely filthy.