tap water results

I agree with Ray that pour-through is easiest method to look into the root environment and it is more likely to be more relevant than pH of water or fertilizer. I should add that it is probably better to standardize the amount of water (e.g. 1/4 cup etc) you apply for the measurement.

Practically, if you are using dilute fertilizer most of the time, pH of fertigation water seems to be pretty much determined by the fertilizer, instead of the original pH of the water. But the pH of pour-through could be quite a bit different from the pH of the fertigation water. Pour-through pH seems to be influenced quite a bit by the root excretion, according to one scientific paper (with Phals). Researchers didn't find any ill effect of having really low pH (around 3.5-4) for these hybrid Phals.

 

Coming back to your question about whether it's safe to use your tap water for watering your orchids, I think it depends some on which genera you grow. Hardier varieties like Phalaenopsis, Cattleya, and Paphs may tolerate 200 ppm ok, but not great. I know from my own experience that Pleurothallids, Phragmipediums, and Oxyglossum section Dendrobiums require much lower TDS than 200ppm for everyday watering needs. They require water in the 50 - 125 ppm range to be happy and lower in this range is generally better. Fertilizer dilution at even 1/4 the recommended rate will significantly increase your TDS count (by an additional 100-150 ppm, easily). It's a good idea to check and see what happens to your water quality when you prepare a typical batch of fertilizer. If you haven't already, investing in a $12-15 TDS meter off Amazon can be helpful.