At PoolDocs sticky on adjusting alkalinity he says:

Also, do NOT try to lower your pH all at once; for all sorts of reasons, it's important to do it gradually!
What would those "all sorts of reasons" be?

I add about two cups of muriatic acid to a 15,000 gallon IG plaster pool every day to keep pH in the mid 7's, but we have very high alkalinity and hardness here. That bumps it down about .2 on the pH scale each time, but it's always creeping up by about the same amount each day. I also add about 4 cups of bleach per day, so that is also pushing the pH higher.