For most machining work, machinists usually follow standard tap drill charts based on thread standards like UNC/UNF (imperial) or ISO metric. However, the recommended pilot hole size can vary slightly depending on the material. For example, when tapping softer materials like aluminum, the drill size is sometimes increased slightly to reduce tool load, while harder materials such as steel plate may use the standard chart values to maintain proper thread engagement.
A good solution is to use a combined metric and imperial tap drill chart that lists recommended sizes for different materials. Many machinists keep a printable chart posted near the machine for quick reference.