With modern digital cameras, that practice has become practically obsolete. Believe it or not, everything can be adjusted upon opening the image in Adobe Photoshop CS3. I've always left my camera's white balance settings on auto, and then when opening the RAW file in Photoshop, adjusted it there. It even has the settings labeled accordingly to most conditions so that you can see the photo closer to the actual conditions when it was shot.
Take for example this photo of a caribou from Denali National Park. It was a particularly cloudy and overcast day, so for this photo I picked the Cloudy setting and Photoshop did the rest. You can see the difference between the original shot (top) and the White Balance adjusted shot (bottom). It's not a dramatic difference, but it creates a much more natural look when the colors are where they're supposed to be.
Take another example of Beaver Falls along the Havasu Creek in the Grand Canyon. You can see that the original (top) is a bit cooler in color than the adjusted (bottom) one that has been set to the Daylight setting. The colors are warmer, thus recreating the actual conditions that were present when shooting.
The final example is from a shot from our hotel window in Waikiki. You can see a big difference in this one, even in the histogram at the top-right. Since the shot consists of mostly fluorescent lights on the ground and in the buildings, I used the Fluorescent setting which turned the photo from a murky yellow, to the appropriate dusk scene that we actually saw.
You can see the sky is more blue and the streets are more defined in varying colors.
If you're not fully satisfied with the presets that Photoshop creates, you can always tweak them on your own by dragging the sliders on the Temperature and Hue meters toward the colors that you'd prefer to see. Don't be afraid to play and have some fun with it! Just remember not to get too overboard.
