Hints For Success:
If changes seem extreme, they probably are, and you may be doing damage to the information in the image. Better to make more than one correction than to make one that is too strong and damages the image information.
When in doubt, make the image lighter rather than darkerbut don't go very far. A shift of more than 10 levels without certainty that you are making a positive correction is going too far. There is less of a chance of making an image too light (unless you are dealing with something very delicate, such as a soft, high-key image) than there is of making it too dark. Dark print will oversaturate and tends to get a little murky, and dark images are generally not as easy to view. Lighter/brighter images are generally easier to see.
If you have switched from a CMYK image to make RGB corrections, it is probable that color will remain CMYK-compatible after Levels corrections. It is good, however, to make sure before blindly shifting over to CMYK. Check your image using the CMYK Preview and/or Gamut Warning controls before converting.
If only one channel has a tail, be more conservative in corrections, especially if there is an apparent color cast to the image that should be retained. You always have the chance to go back and cut more. If you are worried about highlight detail, you can actually paint that back in from the original image scan or from a snapshot, or use blending.
To quickly restore a detail loss caused by cutting a tail, use histories or layer blending (Blend If). For more control, you may want to use both: Paint the correction into a second layer, and use the Blend If layer function and layer opacity to control its appearance.