However, my instructor did not teach me this... is this good practice, or one of these bad habbits I've heard him talk about.
There are many ways to skin a cat and fly an aeroplane, as a quick perusal of this forum will reveal!
It's tricky in training because you have to follow a single set of guidance fairly rigidly, otherwise you get in a pickle. Once you get your licence though it's worth flying with a number of different people and generally figuring out what works best for you. Eventually, you'll find a style which suits you and which you'll then develop. As long as you're safe, don't sweat minor issues like this one. I can't for the life of me see what's wrong with trimming on final.
QDM