I didn't learn it in medic school, I learned it from an advanced EMT in the very first position I worked as a Basic. I can't remember if it was brought up in medic school at all or not.
Another trick I was taught was to spike the bag 1/2 way, pull out the spike, squeeze the air out and then insert the spike in all the way. This way if your bag ran out you wouldn't suck air into the tubing. I don't use that very often mostly because I keep a pretty close eye on my IV bag and I'm usually not giving that much fluid. However, I will use it on traumas and codes where crap is hitting the fan and I know I'm going to be spinning a little bit.