I'm a newb still but I definately notice if the ph is higher like closer to 6 the plants can take a higher ppm of food but at around 5.2 I've noticed that the ppm tolerance is much lower although even at a bit lower ppm they seem to grow better

during a res change you have to compare the ppm before you replace with the new water any difference between those will cause a drift depending on how different the ppm

I have topped 1 footers in first day of 12/12 and I started them at just 500ppm maxibloom with ph at 5.2ish and last time I was at 800ppm to make them happy with 5.8ish ph

I just try to pay attention the first day of a res change and after that ya kinda know which direction and rate it's headin...but I do think there is some subtle connection between the life cycle and the excretion of the plant that causes ph to change I'd say if yer havin issues with ph travel put it to 5.5 and you have 1/2 a point to wander before it's any real issue on both sides ;-)