A dish can become too salty without a single pinch from the salt jar. Stock, canned ingredients, mustard, and soy sauce all contribute, and their effects accumulate.
Start with a small amount of soy sauce in a broth or lentil filling. Stir and taste a representative mouthful. Adding more is easy; removing it is not. Reduced-salt versions still need tasting rather than automatic doubling.
Ordinary soy sauce often contains wheat. Tamari may be useful for some wheat-avoiding cooks, but check the actual label and suitability rather than relying on the name alone. For a guest with dietary restrictions, keep the bottle available so the ingredients can be checked. A dark, savory sauce in an unmarked jar gives them little useful information.