I think it's a good idea to at least know the starting ppm of your tap water, because it'll tell you if your water is crap or not. Just about anybody in your home town or area that grows hydro knows the ppm of the water coming out of the taps. The least expensive way to find that out would be to go over to the hydroponic forum and give a holler out to anybody in your area and simply ask them if they know the ppm of the tap water. I live in the PNW and the water here has a ppm of .007, but in other parts of the country, tap water measures 700 ppm. When water is that bad, people generally use RO or filtered water.
You need to at least know the starting ph of your water and soil. If you are going organic, you then adjust the ph of the soil with garden or dolamite lime so it registers between 6.5-7. Lime will adjust the ph of the soil rapidly. Additionally, ground up oyster shells, ground coral, or ground up eggshells added to the mix will act as a slow release "ph up" and keep your soil from becoming too acidic over time. In other words, oyster shells etc. act to stabelize and buffer the soil's ph and prevent sudden ph swings that can really do damage to your plants. It is pretty much impossible to diagnose any nutrient plant problems unless your soil is at the proper ph. That's because "nutrient lock up" occurs when ph is above or below the ph sweet spot of 6.5-7. For instance, with an acidic ph of 3.5, your plants can starve to death no matter how many nutes you feed to them.