I've never had a single issue with string slippage using the original Kluson / Gotoh Safety-Post style tuners across multiple guitars and with over 20 years experience.
As per jpmist, be sure to put in a sharp 90 degree bend in the string (push the string all the way into the hole first) where it meets the post, use 2 to 3 turns on the wound strings and between 4 and 6 turns on the plain strings and always wind the string down the capstan making sure to keep each string nice and tight when you do so.
Using this method, I get a perfect slip-free result every time!
Last edited: Feb 20, 2018