It's not safe to use listtransactions this way.
I know I've been criticized for being reluctant about listtransactions. Let me explain my reluctance.
The model where you assume you only need to see new transactions because you've already seen previous transactions is not true. Old transactions can change at any time.
Now that we have the Accounts feature making it easier to do it the right way, we're better prepared to have listtransactions.