The importance of a mallet's feature in sheet metal work is related to its function and effectiveness in shaping and forming metal without damaging it. Let's analyze the provided options:

1. A head made of wood or rubber: This is the correct feature for a mallet used in sheet metal work. A head made of wood or rubber absorbs shock and gently works the metal surface without marring it. This characteristic is essential when forming or flattening metal sheets, as it avoids leaving dents or scuffs that a harder material might cause.
2. A heated striking face: Such a feature does not apply to mallets used in sheet metal work. Instead, heating might be relevant in processes like forging, which involves different tools.
3. A built-in measuring scale: While a measuring scale may be useful in some tools for precision work, it is not a characteristic related to the basic function of a mallet in metalwork.
4. A sharp steel cutting edge: A cutting edge is more appropriate for tools designed to cut, such as chisels or shears, not for mallets which are for impact and shaping.

Therefore, the most suitable feature for a mallet used in sheet metal work is A head made of wood or rubber. This aligns with the requirement of altering and shaping the metal without causing damage to its surface.