When casting remote attack spells...

[SlipperyJim]

Just adding to K's answer:
Penetration bonii only matter for spells that have a MR check.

Spells that do direct damage -- Fires from Afar, Flames from the Sky, and Murdering Winter -- derive no benefit from a penetration bonus.
Most remote-summoning spells -- Earth Attack, Infernal Disease, Manifestation, Winged Monkeys, and Send Tupilak -- derive no benefit from a penetration bonus.

On the other hand, I don't think that Mind Hunt gains any benefit from additional levels of Astral magic beyond S4. So it really depends on which spells you plan to cast....