For the first few years after I arrived in St. Gallen, ticket inspections on buses were a rarity. It happened less than once a year, but even then, only on the most popular bus routes. Someone at the bus company must have woken up though, as they now check frequently on most routes. I am getting checked once in every two or three journeys now.
The penalty for no ticket is CHF 100, and if you have forgotten you ticket, it is CHF 5 plus the annoyance of lots of forms and a trip to the main station.
Postbuses it seems are still exempt though. I have still never been checked on a postbus!