Ive already got the GH ph up and down

let me make sure i am understanding correctly. adjust my ph down til my runoff is about 6.5.

but is it better to keep the water phed low or try to correct in soil too, reguardless im goin to have to ph water down every time
You will have to adjust your tap water BEFORE you flush the soil, and everytime you water/flush you will have to adjust your pH since your tap water is so high. So your going to be testing the tap water before you adjust each time, and then test the soil run-off water when you do the flush, so the min testing you will be doing is 2 per plant.

If your thinking of creating some water that can be used anytime, kinda like a reserve in hydro, your gonna need to keep that water from getting any light, put an air stone in the water connected to an air pump to keep the water fresh (this will be when you have nutes in the water). If you dont have nutes in the water you dont need the airstone and such since the water will be going into soil and the roots already have air.

The good side of this is that with soil your only looking at having to do this every 3-4 days depending on how well your soil drains. Flushing schedule i have decided to do once a week, others do it once every couple, but i want to keep the soil clean and only nutes in there. I am sure i am loosing some nutes with the flush, but i am running around 1500ppm in both my DWC hydro and soil plants, so they always have the nutes in each.

I think with the pH down of the tap water you wont need the other crap that you put on the top of the water, especially if your doing a regular schedule flush. just my opinion and others may thing different, but if i can control the pH of the water going in, and my soil is as consistant as possible, no time release crap nutes or other stuff added, then i know where my pH is in the soil at all times and it makes the grow ALOT easier.