What are you growing in?
You should always feed/water at the appropriate level. In general this is usually 5.8 for Hydroponics and 6.5 for Soil.
If you are in soil, use a 6.5 solution all the time. If you are concerned about the pH you can use several gallons of a 6.5pH solution to flush the soil and balance the pH in that way.
If you are in Coco, or Peat, then you should be using a 5.8 solution. The run off of coco is usually pretty high due to the buffer of the media. To get a more accurate representation of the pH of your media you can take a scoop of the soil from a few inches down. Make a slurry with some filtered water and test the slurry.
Otherwise all you really need to do is get a lot of run off and keep your pH right where it needs to be. Some plants will like it higher during some periods, lower during others, so listen to the plants and keep a good journal.