water ph question

I have noticed whenever i change out my nutes I start with water PH'd around 7.2.When i add my nutes, mollasses and cal/mag my PH drops to as low as 4.8 sometimes lower.I use R/O water and i only add just enough nutes as recommended by the bottle.I dont use Ph up because after 2-4 days the Ph levels off at the 5.8-6.2 range, but in the mean time my plants have been drinking the low Ph'd water.
I grow in pea gravel and coco croutons.The guy at the hydro shop told me to watch my PH using pea gravel and suggested the coco to help maintain PH balance.
My question is will it hurt my plants to drink the lower PH'd water for a couple of days while the Ph bounces back or should I use the Ph up/down and control the Ph all the time?

The best would be control at all times, when you test the pH is it right after you add nutes. If so shake and let sit for about an hour and and then test to give time for the nutes circulate through out the water. I would try and only give the plants as low as 5.4 so that they don't get to nute blocked by the low pH for those couple of days.