i have never worried about it.
i knoe from experiance that a 100% water change can kill some/most of the fish in your tank. mabee it is the chlorine, i dont know. mabee it is something else? a 50% water change most fish, seem to like. even straight out of the tap.
i know that everyone knows this, but it is worth repeating... change your water often, and in small amounts.
when you catch your fish from the wild and put them in your aquarium, the different water will be a huge shock to them. i always keep this in mind. a completely bare quarantine tank for at least two weeks, preferably, four, is/should be mandatory as well. i hate white spot. yukkk!