Do character saving throws (Fort, Will, Ref.) scale like the the rest of the skills in the game. So it is your level+ability score mod.+ the other variables.
As far as I can tell, the only things that don't scale +1 / level (ie don't use proficiency) are:
- Damage
- Attributes
- Hit points (kinda, they do go up evenly per level, just not based on proficiency).
- Gold!