If you're only feeding them straight tap water, I wouldn't be concerned at all. If, as m0use rightly suggests, the pH is consistently falling, it USUALLY means the EC is too high and you'd need to back off on your nutrients... but if you're not giving them any nutes, there's nothing to back off from! If you're still feeding them (and why wouldn't you be?) the trio, I would suggest backing off the Grow by half... plants, in flower, don't need as much nitrogen as the Grow is giving them and this may be leading to a lockout of other nutes which your runoff pH is trying to tell you. But for me, if the plants are looking healthy (and they are), there's nothing to worry about...
Good luck!