I ph'd my water when I started before I added nutes. Couldn't afford the meters and ph'ing with a drop Kit after adding nutes was impossible. THe correct way is too add the nutes and then ph your water. Many of the nutrients that have been developed have ph buffers as a part of their makeup, they could have different effects when mixed with other ph buffers. This is why you want to ph after adding all of the nutes, etc that you are using.
When I could afford the meter and started using it I would check my starting ph, usually around 3 (I have R.O.) after adding nutes it would usually be around 6, sometimes less. If I had added ph up before adding the nutes I would have to use ph down after adding nutes, not good. You are right to ask questions, thats how we learn, welcome to the forum. VV