A couple bloggers have posted mechanics for true names. Or maybe it was just Beedo posting two different articles, and that gave me the impression that everyone was doing it... Anyways, I personally would want to avoid adding too many specific modifiers or penalties, like a -4 to saves, so I wouldn't use a mechanic like that. Nor would I want to add an extra roll. But I might use something else...
Say: knowing the true name of someone lets you limit an area-effect or random target spell to just the people you name in your incantation, so that you could (for example) hit a demon with a fireball without risking hitting your friends... and it lets you cast a spell targeting the named individual over any distance, as long as you can see them, somehow. And additionally, knowing the true name of a spell caster lets you shake off the spells they cast (free Dispel Magic, even for Fighters and Thieves.)
The limit on all of these would be that your Intelligence must be greater than the level or hit dice of the being named. This is meant to reflect knowing the precise pronunciation of the name.
The drawback is that speaking a true name is something the name being can feel, like "someone just walked over my grave." A spell caster or supernatural can even track someone who speaks their true name, although they won't know who the speaker is or how powerful they are; use a standard 5+ on 1d6 roll, add a bonus for high Intelligence, and add +/- 1 depending on whether the named creature has a higher level or not.