I believe that the alk in natural sea water is 7. I personally like to keep it steady at 9.
I like to mimic the ocean as much as possible in my system. The reason for 9 is that if it dips a little for any reason it will still be above 7.
I was told between 7 and 11 is ok. Also some people believe higher alk gives you better growth. I don't know if that's true though.
I have seen beautiful setups where absolutely nothing is tested for only alk. With that said, alk is the most important parameter in my system and the test I do most often.