Selecting the right filler metal is a critical step in welding procedure development and should never be treated as a default or trivial decision. This article explains why matching strength filler metals are not always the best choice and how undermatching filler metals can improve ductility and reduce cracking, especially in highly restrained or high-strength steel applications. It also explores how service conditions—such as low-temperature performance, corrosion resistance, stress relieving, and wear requirements—directly influence filler metal selection. Real-world examples illustrate why tensile strength alone is insufficient when evaluating filler metals for production welding. Understanding these factors allows welding engineers and practitioners to make informed, economical decisions that improve long-term weld performance.