Remember that Shed's original thread had a lot to do with the Promix that did profoundly become affected by the fluids that passed through it. A good organic soil, especially one that is recomposted constantly, is a lot more resilient to that and indeed is why I supplement my soil with raw nutrients and buffers in between grows. Over the length of a normal grow, the soil is not going to change enough to even become a factor, because a good soil like FFOF is buffered at both ends, with peat at the low end and dolomite at the high end. Don't expect your soil to become overly acidic over time, unless you are looking at a 3 or 4 month bloom... then it might become a factor that bears watching, or in permanent locations, such as a tree that you are trying to nourish.
Yes, you can use tap water with no problems in a soil grow... and a lot of that ppm is actually beneficial, such as a bit of magnesium. People that use tap water used to not usually have magnesium deficiencies, until LEDs came along. Don't worry about the ppm of your water... it is going in soil and that number is meaningless. Even the fear of chlorine in tap water is really mostly myth, and surprisingly our plants actually need a tiny amount of chlorine. I have heard it said that it would take swimming pool strength chlorine to actually kill our microbes... the are tough little critters.
If you are using bottled nutes, they have a specific pH that their chemical compounds are designed to break apart at and become mobile in the solution. Whatever Shed is using, it is good stuff, and it must have a nice wide range, plus Shed has learned how to manage his water so that his pH is never too far out of line... plus he uses up the buffers in his medium to eliminate the need for adjusting pH. Soil is a little slower to react to an improper pH, no matter how well it is buffered, and it becomes more important to hit the target a little closer to center. In a fast soil, coming in at the high end can quickly make some of the elements immobile, and it is necessary to jump around a bit to make that happen. There are all sorts of tricks to make that happen occasionally, such as water/nuteing without pH adjustment, taking things to the low end to be buffered back up, and then coming in with unadjusted water the next time at the high end to "balance" things out... I think it is smarter to know why pH is important and just adjust it to the right spot so as to not waste nutes, if nothing else.
Once you wean yourself from that bottle, then tell Shed you no longer adjust your pH... or do what he is doing and get some Promix... apparently it is good stuff.