We would prefer feet first since it gives a sense of reassurance to claustrophobic patients when informed that they would go with their feet in first, but nevertheless the scan can be performed in either directions. Some scan makes have an effect on the peak SAR values based on the landmark used and anatomical area scanned as in below paper.
The limits for the maximal SAR depend on the patient position inside the scanner. If the position is not known, the lowest SAR limit along the body, which is in the neck area, is to be set to the global limit . Determining the patient position within the MR scanner enables to impose an SAR model, adaptive to the body region, and consequently, to exploit the maximal image resolution