Keep precision during the math
Do not round an intermediate value merely because the display looks long. Early rounding can move every later result.
Match the measuring tool
A tape measure, ruler, caliper, and micrometer do not support the same resolution.
Match the material
Paint coverage does not need the same precision as a bearing fit. Wood movement may be larger than the decimal you are trying to preserve.
Round at the decision
Round when choosing a drill, marking a scale, buying a container, or setting a tool, not automatically after every formula.
The calculator is one part of the job
Use the result to support a decision, then compare it with the drawing, material, tool, product instructions, and actual conditions.