Some things, like weapon critical effects, reference your class DC, but most casters do not list a class DC progression.
It would however be interesting for example for a Magus using Arcane Fists, as a critical hit with a brawling weapon calls for Fort save against your class DC to avoid slowed 1.
Instinctively we have been using the spell DC, because for most classes that have bot (Champions for example) these move together, but recently we discovered that it is against the rules.
The only rule I could find states that everyone has it, but does it increase for a Magus?
(Magus is just an example, a Psion could also be using an ancestral weapon with the 5th level feat or an Owlbear Claw)