It's important to know what salt brand is being used by the store. If they are using a salt brand that mixes to a lower alkalinity, then you shouldn't have anything to worry about, because stability is more important than the number itself, as long as it's in acceptable ranges. I use Tropic Marin Pro which mixes Alk to about 7.5, so, my Alk is normally around 7-7.5 dkh.
Just for conversation, lets say hypothetically speaking, the store uses a salt that mixes to 9/10, and every time you do a water change, you're dosing alk and bringing it up a bit, which isn't really an issue. But, then one day, you're battling cyano, or algae and decide to a large water change- well, you would have effectively shifted those parameters far off what your reef is used to compared to normal scheduled water changes. This would also affect you if you have to do a series of water changes if you accidentally contaminated the water for example.
I would recommend getting a RODI filter and making your own salt water. The initial cost will pay for itself pretty quickly depending on how big the tank is.
