In Taiwanese "3-7" refers to the 30-70 division of money between a prostitute and her pimp, so it cannot be lightly employed. The fondness for "eight" comes, most people would say, from the Cantonese.
閩南語裡的「三七」，講的是與妓女三七分帳的皮條客，可千萬不能隨便亂用。對「八」的嗜愛，一般認為是始於廣東人。