Smoking cocaine never became mainstream. It remained a part of drug sub-culture. It was propagated among cocaine users who were looking to get higher, and could afford to smoke obscene amounts of cocaine. It began in 1974. It was isolated to Southern California, in L.A. and especially Hollywood. The first related hospital case was in 1975. The greatest dope dealer in L.A. at the time was Tootsie Reese. He says that he learned about base in 1976 when he visited his "white friend" chemists at UCLA Berkley. A year later, he knew about using baking soda.
Since 1975, "conversion kits" had begun being sold in magazines like High Times.
The thing to realize, though, is that cocaine was still a boutique drug. Basers consumed large amounts of it. It was mostly practiced by entertainment professionals and business execs. Richard Pryor's accident brought basing to public attention.
A street manual from 1979 does not include baking soda. A Rolling Stone article from 1980 says that kits contained Ammonia or baking soda, and were added to ether. It can be presumed that the ingredients of the kits were not disclosed to the buyer. The article says that free base was not sold but made from street coke. For some time, though it could have been a trade secret among the dealers or kit makers. Free base cocaine is not as stable as Cocaine Hcl, but crack doesn't exactly sit around.
When cocaine hit the masses in the 80's, it probably became common practice among free basers. These were people who could afford to buy cocaine and make it themselves for the sake of purity. Crack dealers would probably have continued to use the old Ammonia and Ether formula for efficiency. Regardless, the scourge of the rich became the plight of the poor.
I just wrote this whole answer, and now I realized that baking soda has nothing to do with safety. It was switching from ether to water. While it seems simple, it does not appear to mentioned at all in the period mentioned (up to 1980). Nor was it easy for the average American to find cocaine.