If your valves still look like this:FWIW, I still turn my knob back a quarter turn and teach it that way to students. A valve all the off or all the on feel exactly the same. A valve a quarter turn off, feels "loose" and easily differentiated between all the way off. Yeah, I check all my students' tanks before we splash.
then yes, you have to turn it a bit back. These old valves were a nuisance, the construction may cause the valve to jam when fully opened, due to expansion of different metal types inside the valve, caused by the decompressing air which draws heat from the surroundings. Alec Pierce explains this on YouTube.
These valves were used in the previous century. Nowadays, valves have the knob on the side and are constructed differently. Opening the valve all the way with your fingertips is the correct way.
Reason for using your fingertips and not your whole hand:
Inside the valve, on the stem, is an o-ring that does all the work for you, keeping the air inside the tank and preventing any leaks. If you force the valve fully open, that o-ring is squeezed between the stem and the teflon ring inside the nut.
On the left you see what this o-ring should look like. On the right you see o-rings that are worn-out by using force to open tanks.
Fingertips
All the way open
Even if instructor Pete says "quarter turn back" (and I hope your students read this ): all the way open