Large swings are more of a concern, if your tank is say 8.4 at its highest just before lights out & drops overnight to say 8.1 thats fine, if it drops more then that would be a problem, the same as a 8.0-7.7 would be ok.
I have changed salt brands 3 tim,es now, always done 10% water changes per week & so far haven't hurt anything............
Cheers Shelton.