Whether you use -50 and +50, or some other value, is dependent on the specific image, lighting conditions, camera, lens, and individual preference. Very high dynamic range images with deep shadows and bright highlights may require higher than 50 settings (-60, +60 up to -100, +100) to reveal detail in those areas. Subjects with no deep shadows or bright highlights (that is, overcast lighting or fog) may require lower values (-40, +40 to 0, 0) to maintain contrast. In fact, these are the types of images that may require using +Contrast, +Clarity, or +Dehaze. However, after applying those controls the image may benefit further using a small amount of -Highlights and +Shadows. Again, this is due to image adaptive behavior of the Tone controls that causes them to interact with each other.
The Dehaze control is useful for images with visible atmospheric haze (fog, smog, smoke) and images with low contrast due to lens flare or other lens issues. In the latter case, adding a Dehaze setting of 15-25 may be just what’s needed to add some life back into the otherwise flat images. Atmospheric haze often requires higher settings and may also require using the local controls (Adjustment Brush, Graduated Filter, Radial Filter) to add additional Haze control in areas requiring more correction.