I'll be using the regular Saving Throws as per their category names — i.e. Paralyzation, Poison & Death Magic vs. paralyzation, Breath vs. breath weapon or other area-effect attacks, etc.
However, for more general-purpose saving throws, such as clinging to a tiny crack in a cliff face to avoid plummeting to a horrible splatty doom, I will generally just use characteristic saves — STR, DEX, CON etc. — because I can't be bothered remembering which of the BtB saves are supposed to be used for each of those. I will graciously allow the PCs to add +1 to their CHAR saving roll for every band of 3 levels* they have, because I'm a big old softy.
They also get another +1 for saves based on their primary stat of their character class — i.e. on STR rolls for fighters, INT rolls for wizards, DEX rolls for thieves, etc.
- * i.e. +1 for levels 1-3, +2 for levels 4-6, +3 for levels 7-9, and so forth.
