a lot of other things can botch a negative besides exposure. since I'm an imprecise weekend photographer, I usually take 2 identical and slightly overexposed (sheets in B&W. based on a shadow spotmeter reading, and use the second sheet to bracket development in case I get it wrong the first time.
color transparencies are entirely different. there I'll rate an EI 100 speed film at 80 and take an straight incident reading or spotmeter the brightest value in the scene and place that at "zone" 7.5. As David said, with transparencies, the nuanced difference with a small bracket arount correct exposure is usually important.