Answer: I apologize if I was not clear. Your purple fountain grass will not survive the winter in your area, so it will be dead by spring no matter where you cut it.
If you had some other hardy perennial grass (a grass that is reliably winter hardy in your zone) instead, then you would cut it back to the ground, just above the soil. The stems and top growth die back over the winter and must be removed. The cutting is done in late fall after freezing weather has killed the stems, or the following spring. The dead material is removed to tidy the plant and to allow sun to reach the crown and stimulate new growth sooner in the spring. The plant regrows each year from the roots.
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