Do I have to back up again after making any transaction? (ie. Bitcoin-qt client auto create change address after making transaction so you have to back up the wallet again)
not sure about the answer to your actual question, but if I were to guess I would say the answer is no because of TITAN.
off-topic, but interesting: for bitcoin (and altcoins that use the QT client) if you open your bitcoin.conf file and add the line:
keypool=aBigNumber
it will go ahead and pre-generate however many addresses you told it to (I chose 5000). It takes a minute, and it will look like the wallet froze. Then you dont have to worry about backing it up for a while.