The answer to your question is based more in practicality than on what is accepted (or unaccepted) etiquette. Manners and customs often change from generation to generation, but the simple fact is that most etiquette-based customs are based in practical reality. Consider that spilling one's soup onto one's shirt, blouse, or lap is not a good thing---then consider the simple effect of gravity. It is safer to spoon AWAY from yourself than toward yourself, to prevent the embarrassment of spillage. One must also consider the context of pre-television and I-phone dinner settings; people used to have conversations during dinner, without the distractions of modern electronics. One could then focus more on the mechanics of eating, hence pay more attention to etiquette as part of the "dinner ritual". Since dinner was often a drawn-out affair punctuated with polite 'conversation, tipping one's bowl became not only evidence of not being in a rush, but a reflection of proper care taken when eating. Emily Post was merely codifying what was common practice in "genteel" society---but this practice was also grounded in reality. Victorian Society was part of this movement. There was no definitive and widely-accepted rule that I know of until Madame Post established written suggestions...but she merely published what was already polite practice in middle and upper-class settings. As a side note, the very word "supper" is derived from "sops", which what people often ate for the day's last meal: chunks of bread soaked in whatever "soup" was left over from the day. So "sopper" became "supper"...before there was even a wide use of spoons!
