The answer is straight. Milady's standing-posture guidance states this directly, alongside other rules like keeping your chest up and out, holding your shoulders level and relaxed, and not slouching.
The book treats this as part of a mechanically correct standing posture, distinct from sitting posture (covered separately, with its own set of rules like keeping the hips level and horizontal). "Straight" is the specific word the book uses for the spine — not relaxed, curved, or elongated.
Milady Standard Barbering, 6th ed., Ch. 3 (Professional Image), p. 45: "Stand with your spine straight."
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