Our eyes are extremely forgiving about color balance — most scenes look fine, whether the source of light is sunlight, cloudy sky, tungsten, fluorescent — even if these light sources are mixed. But cameras are not forgiving. If you shoot in RAW mode you can simply choose the color later in computer — but only if the light sources are not mixed. (With JPEG you can still shift the color in computer later, but that brings along some quality issues.)
Understanding color balance is another area that separates many professionals from amateurs. It is not that tricky: if you shoot with reddish light, you get reddish pictures, which maybe you can fix in computer. But if you shoot with mixed reddish and blueish light, it is hard to fix both colors later.
It can be very useful to shoot something gray or white — especially if there is nothing else in the photo that you can identify as neutral. I carry around a little gray plastic card to check color, and I use it, though not often enough.
Bottom Line: Be aware of the differences in colors of light. Use them as a creative tool, or fix the problem with gels.