Passive feats...
FvS gives +1 to spell pen
Wiz gives +2 to spell pen
Both can be stacked x3 giving +9 to spell pen if you TR that many times.
So....
How much spell pen would actually be necessary to always pass the spell pen check?
Without TR, the conventional wisdom is if you want to land any DC spells you need all three enhancements, should work in the two feats and and item is also helpful.
Even with all these on my Wiz/Rogue, I don't always pass the spell pen. The rogue levels don't help but it was worth it. Most of the time, I pass my spell pen.
If I TR my Wiz one time, I can skip taking one of the feats and put something else there.
If I TR enough times to get all those passive feats, I can skip taking the 2 feats and 2 enhancements or take a spell pen feat and skip the enhancements and put those AP into something else. I'll still come out ahead.
Really though, how much spell pen is needed?