Try graphite powder or "nut lube" before you go dropping the bank for a new nut. Seriously, newer tuners are really good units, for the most part, so most tuning issues are more related to strings binding in slots rather than tuners slipping.
On a related note- I replaced the saddles on one of my guitars with graphite saddles- not only did it exponentially reduce string breakage, it improved tuning problems. It was such a good change I did all my guitars.