Tasting dressing alone is useful for spotting an obvious problem, but it cannot tell the whole story. Potatoes, beans, and grains soften the impression of acidity and salt.
Dress one small piece of the food, taste it, and adjust from there. A dressing for delicate leaves may need a lighter hand than one for a large bowl of warm potatoes. Add it gradually so you can stop before liquid gathers at the bottom.
Keep texture in mind too. Thick tahini dressing clings well to roasted vegetables but may need thinning for finely shredded cabbage. Oil-heavy dressing can make soft grains feel greasy if you add more simply to increase flavor.
Sometimes the answer is to season the food itself. A bland potato cannot always be fixed by pouring twice as much dressing over its surface.