Flushing is the process in which you add mass amount of water to "Flush" the additional salts and nutrient build up in the soil. New soil, doesn't have this problem.

However, many soils you can use more than once. 2.. or even 3 times sometimes.. if you're reusing old soil medium. then yes, you should flush the soil before you plant another plant. New soil? there's no point in flushing new soil.