If you are using GH 3-part then 3-part has buffers...And If you put the correct amount of 3-part in distilled water you will be very close to *5.8ph which is the correct ph for hydro.

*I propagate w/tap water from my well, and do not adjust the ph. If you have no roots...Then you cannot hurt the plant with normal water.

So, If you are using just the water for clones or seeds...I would say...Don't add phdown

Otherwise, if you are adding nutes...what nutes? are you using it for?