I don't know that my mom taught us to "spit" - I do know she always bought the cheapest brand possible and there was no way I was swolling it...I do remember in 3rd grade a dentist coming to school - we got toothpast; tooth brush and some pink pellet you ate and it stained your teeth to show you how you were not brushing properly...and a song
up down side to side
the choo choo train will glide
spit, swish, gargle and spit again
now SMILE!
I tought that same thing to my kids - more than the spitting part - I always have wondered why a song that didn't really rhyme and made no sense has stuck with me!!