We all develop bad habits. Free wheelin' is one, but unless you have your head up your arse, it's not dangerous. I'm surprised he doesn't have you start out in 1st.
I float the gears so it's pretty easy to downshift, but it does take more fuel to do so..... so I usually don't. I'll downshift a couple of gears, but downshifting to a stop is just extra work. That is why there is a middle pedal.
I don't wear my seatbelt in the yard, or when backing. It constricts me and I need to move around to really see in the mirrors.
Your mentor may be trying to train you in the "right" way to do things, but I could make a safe bet that he doesn't practice what he preaches. If he's an annointed trainer, you can bet on it. If he's just a average joe city driver, he might be trying to impress you with the difficulty of driving. Whatever.
Once you're on your own, you can do what you want as long as it doesn't get you in trouble. My trainer told me that he had to show a problem, and as the training went on, a resolution to the "problem". It's window dressing and everyone knows it.
If you are on your own now, congrats! Shift like you want to.