Manual kelvins, always. I usually know beforehand in what range of kelvins I would be standing on, but its hard to actually put your finger on it. Why? Because if you're lighting all up with tungsten lights (3200 kelvin), even if you have a preset for it, some light bulbs may be worn out, which will give you a warmer tint. If the voltage is not steady, or steady but not on the mark (220v or 110v depending on you country), the lights will have a warmer tint as the voltage drops. Happens a lot when working with generators. If you have a dimmer on a light, as you dim it, it also changes the color temperature slightly.
As for CFLs (3200/4000/5600, there are a lot of options), you have to consider that the white color is actually painted. The tube/bulb is kinda like thouse black/violet lights from discos. So different manufacturers, different batchs, different dim levels, equates to different color temperatures.
Regarding daylight (5600 as a standard), color temperature changes all the time. Sunset/sunrise is a lot warmer, cloudy days are a lot cooler, etc.
So, going back to the beginning, knowing all of this can give you an approximate idea of what is the color temperature of your situation. BUT, there is another factor: the camera. No camera sees everything the same as the next, even if we are talking about two Canon 7Ds that were manufactured one after the other. It never happens (color grading in post is very important). And if you put "5600" in the camera, maybe the software wasn't calibrated perfectly and you'll have to set it to 5800. It happens.
The best thing you can do, is play with manual kelvins and learn from it. All the time. In every situation you're on, try to see what changing the WB does to the image, and draw your own conclusions from that. White balance can be just a setting, or it can become a tool.
Hope it helps!