It's always best to get it right (lighting, WB, etc.) the first time then make minor adjustments later becuase the more adjustments you make the more data you're likely to lose. E.g if you have to crop a picture, you would lose detail in enlargements.
As for CWB being a waste of time... I disagree as well (though I've only used it a handful of times). Auto White Balance doesn't always turn out right (unless that's the particular effect you're looking for), and adjustment may be required.
Also, AWB can 'balance' differently even under the same lighting conditions - i.e. it isn't always consistent because it reacts to the lighting. By fixing the WB, be it custom or daylight or something else, you get consistent colour even if you re-compose in the same light with less lighted areas, etc (whatever else AWB reacts to)
(Anyone know the exact parameters digicams use to auto-decide which WB to use? pretty sure they would defer by manufacturer).