The only times I've had to use my pH up is when i made a mistake and put too much pH down.. My tap water is 6.9pH so to get it to 5.8 I just add like .5cc of pH down.. You might need pH up as the res pH drifts to adjust it as a whole. And when you say it gets more acidic.. Acids are low, bases are high. So if what you say is true and it does get more acidic, you would need pH up to make it more basic But my pH decides which way it wants to drift when it does (not always toward the acid side).. it says 5.8+/-.1 for months at a time.
when i mix my nutes the ph is usually low after everything is mixed, but in my case im using PH up juss as much as i use my PH down. it's always nice to have both. buy both.