I make sure to add prime every time, even if its only a 10% change. I know in some water districts (like mine in northeast Atlanta), the amount of chlorine they add varies. Sometimes you can SMELL the chlorine in the water, sometimes the water seems fine, but given that it varies, just because you get lucky once doesn't mean the next time it won't kill your tank. Seems too big a risk to me to NOT make sure to add it every time, it's not that much more effort or expense, and it's one of those "a stitch in time saves nine" things... I'd rather have to constantly remind myself to add something to my water when I change it than to clean up dead fish.