I find it interesting that many would like to make it sound like an "underexposure" is the end of the world, and a photographical failure.
What that stance significates is failing to understand the implications. I spent quite a few years of my life fighting against this erroneous mindset when educating print and pre-press personnel starting out in their first stumbling steps of the "digital revolution". Back then, pre-press personnel was (if possible) an even more tradition-laden and anachronistic group of people than what photographers are today.
As someone mentioned Adams, I have to say that he is a person you really have to admire for the amount of energy and time he spent really getting to know his medium (film), and how you had to expose to get the most out of your envisioned image. He (as he worked with the physicists and chemists at Kodak) set up some really workable testing rules and also inspection criteria to be used when looking at image results.
And he always tried to get the exposure "right".
Now try to understand the implications the sentence in bold above sets. Getting an exposure "right" is NOT the same as trying to make 14-16% gray as you see the image conform to a certain photometric exposure at the film surface! And he never claimed that it was. If there is more pictorial detail that you want to emphasize in the end result (for him; film, development, print) in the shadows - you should really overexpose - compared to the baseline scene average exposure. And if there is "important detail" in the highlights, you need to underexpose [compared to the reference exposure]. He accepted that there was always a compromise between highlight, midtone and shadow local detail. I would recommend a visit to his old home, if his son still has the tours he used to. If you're nice, to him you might even get to see some "behind the scenes" notes and development plans - they're really impressive in attention to detail, and shows how much he mauled his negatives, using both over- and under-exposure, to get "what he wanted" out of them. Regional corrections of + or -3Ev was nothing out of the ordinary either.
In digital, the entire ramification of the process has changed.
Film has more latitude, but much less usable DR within a fixed scene
Film has an bell-shaped MTF modulus, digital is linear
Film clips and blocks gracefully towards both highlight and shadow; and
Digital clips hard against highlights and fades gradually into noise in shadows
This means that you can't treat digital like you did with film, and the most obvious differences are:
a) as long as the entire scene can be contained within the digital DR, you can put the raw-file related exposure (which is not the same as photometric exposure!) wherever you want - it does not effect internal detail contrast.
b) you have to mind the highlights (more) with digital - since when they're clipped, all internal detail contrast is GONE. Impossible to retrieve unless you let software guess what the clipped channel values should be from surrounding color information. This only works for about +0.5Ev from initial clipping.
To underexpose the raw file - again, this is not the same as the photometric exposure! - is the only practically available method to reign in the highlights in the scene. And this is also what Canon does with the "Highlight Tone Priority" or "HTP" option when you shoot jpg. The camera will underexpose the raw file, and then bring image midtone brightness up again - but with smother upper S-curve end of the brightness curve.
Underexposing an image that doesn't have any highlight areas (zones? maybe applicable) you want to protect though - that's only stupid. It hurts image quality.