In a plantless cycle, ammonia is turned into nitrite as bacteria grow to eat it, then on to nitrate as more bacteria grow to eat nitrite. Both nitrite and nitrate are acidic so they tend to make the pH drop.
Tap water has some carbonate in it; it comes from the rock the water flows over. Some places have a lot of carbonate because the reservoir or river flows over a lot of limestone. Other areas, like Northumbrian Water's Kielder Water doesn't sit on limestone so there is not much carbonate in the water.
Carbonates react with acid. If there is a lot of carbonate in the water, the amount of acidic nitrite and nitrate isn't enough to use up all the carbonate so the pH stays constant. But in places with very little carbonate it can all get used up and then there is nothing to stop the acids dropping the pH.
The problem during cycling is that our filter bacteria need carbonate to grow properly, and they need an alkaline pH. At below pH 6.5 they stop multiplying. And if there is not much carbonate in the water there may not be enough for the bacteria to grow well enough to multiply.
So in places where KH in tap water is low, it is easier to add bicarbonate of soda to artificially boost the amount of carbonate in the water. All the sodium in bicarb (the soda bit) is not very good for fish, but we do a big water change at the end of the cycle to remove all the nitrate the cycle has made and that also removes bicarb.
My KH (carbonate hardness) is 3 degrees. When I did a fishless cycle (no plants) in 2013, my pH dropped from 7.5 to below 6.
But when there are plants in the tank, they use ammonia as fertiliser. They don't turn it into nitrite or nitrate so there won't be as much acid formed. There will be a bit because of plant waste (yes plants do make waste just not on the scale of animals
) so there is less risk of a pH crash than when there are no plants.
I didn't test my pH in 2013 until I realised that the cycle seemed to have stopped and that's when I found the very low pH. Your cycle with plants may never suffer a pH crash but as pH is the simplest of all the tests (no waiting 5 mins for the colour to develop) testing for pH when you test for all the others will catch the pH starting to fall if the does happen.