A professor at my university did an experiment to determine exactly when you should water a plant. It probably differs by plant, but he came up with some very interesting results. A good soil is comprised of about 25% water. What he did was test the rate of photosynthesis as the percentage of water in the soil declined. He did this by having each plant in a completely sealed environment and measured how quickly CO2 was changed into O2.
He found that the rate of photosynthesis was relatively steady as the percentage of water fell from 25% to ~14%. Once it hit 12.5%, photosynthesis fell dramatically, to practically nothing. What is most important, however, is that once it reaches that point, the plant can never rebound back to its maximum level of photosynthesis. It can recover and still be a healthy plant, but it will never achieve what it could have if the soil didn't reach that stage of dryness.
I don't know exactly how his results would apply to cannabis, but I thought I would share that little gem of information. You can buy probes that measure the moisture content of soil, so if you had the know-how you could set up an automatic watering system that watered when the moisture content was around 14% and stopped at 25%. It would take the guesswork out watering, which I guess would be more beneficial to new growers. Master growers obviously wouldn't need such a system, but maybe one could try it out and see how the crop compares to his normally-grown crop.