You don't pH distilled because there is nothing with which the acid could react with. RO has around a "natural" pH of slightly acidic due to atmospheric CO2 that makes it into until a certain equillibrium is reached... but it doesn't push your medium up or down it's basically neutral.
You shouldn't stick your pH probe in RO as this will force some of the inner electrolytes out and make it harder in the future to get a proper reading.
You gotta watch drain pH to get some idea of what your media pH is - this is important for proper nutrient absorption. IMO you should aim for 6,2, usually cocos would be handled a bit more acidic, and soil can go up to 6,5, even 6,8. Roots of plants, microbes and N-metabolizing can still alter substrate pH, usually drag it down acidically.
You can see how much N a plant has by the greeness of its leaves. In abundancy they will get darkgreen, then start to claw downwards beginning with the topleaves. In deficiency, the plant will first generally brighten up a bit, later the lower leaves will turn yellow and fall off. The availability of nitrogen has the greatest impact on the overall growth of a plant, so this decides if, and when, you should start adding extra liquid fertilizer.