Under normal circumstances, the margin of a dental restoration should be placed supragingival because it allows for easier cleaning, better periodontal health, and improved longevity of both the restoration and surrounding tissues. Supragingival margins minimize plaque accumulation and reduce gingival irritation. They also facilitate accurate impression taking and better visual access during preparation and restoration. Subgingival margins are reserved for specific indications such as esthetics, caries extension, or restoration retention when necessary. Thus, maintaining the margin supragingivally whenever possible promotes overall oral health and clinical success.

Reference: Sturdevant's Art and Science of Operative Dentistry, Roberson, 7th Edition.