If I change less than half I only put in enough dechlor for what I'm changing, if I change more than half I use enough for the whole tank. I have no idea why other than this is what someone on another forum told me to do when I set up my main tank and it's always worked.
In the "old days" before chloramines people used to age their water, but I really don't see the point now. A water conditioner like Prime is really effective and also cheap to use.
I agree to making it as easy as possible.