Should I overstack res for penetrating bosses? or just vs curses?

how many bosses penetrate ?

i didnt deliberately overstack my res, so if it dont help much at all
ill just replace gear with a goal of 75 all.

You need to overcap for both curses and exposure unless you are immune to both. There are other good reasons to overcap but those two are the most pressing I can think of.

Overcap does not matter for penetration. To protect your self against pen you have to change the final damage type to something else. For example if something has fire pen (atziri flameblast as an example) you would need to take fire damage as something such as chaos.

As to which bosses have pen Im not sure of a complete list.