When it interferes with rp rather then facilitates rp is my answer, granted i know that is horribly imprecise and will differ for each game but that is also the truest answer I can think of.
Yep. And it's easy to have a 'too much' situation where adding more helps: eg, you've got obfuscate code, but no code to allow people who ICly can see through it to see through it. Either don't code obfuscate and have everybody roll and ICly behave as if they really can't see it if they fail, or add code to give them as have the right powers an automagic chance of seeing through it.
I once had a character who was shot with a shotgun his first hour on the grid. The PC who shot him said that he wanted to hit him over the head with the gunstock, not shoot him, but was told that the code demanded that he shoot once he'd wielded. I think in that particular case the GM was just being silly, but that sort of thing does happen. It really mucks up the game for me if I'm told I cannot do something that's possible in the imaginary world because the code won't support it.