The thing is, tieflings are good at Bluffing, and they are fairly scary to normal humanoids. They also, seemingly, would excel at sorcery. Use Magic Device? If anything, as denizens of Sigil I think they would have a knack for making magical trinkets work. Their only real issue is Diplomacy. You could give them a racial penalty to Diplomacy, reflecting their furtive, perhaps creepy nature, but is beyond what's adequately covered by, "Eww, a tiefling, I'm ... unfriendly?"
In my view, the tiefling Cha penalty is a holdover from AD&D. In AD&D, there were no sorcerers. There was also very little in the way of social skills. Thus, Charisma was basically reaction penalties and hirelings. As rogues and wizards, Dex and Int bonuses made sense.
In D&D 3e, the only tiefling traits that would really carryover for a low Cha would be Diplomacy and their Leadership score. In every other way they seem like high Charisma creatures: they are supernatural, associated with the Sorcerer and Warlock class by heritage, good at Bluffing, slightly frightening to most humanoids, and probably able to be more forceful with summoned creatures and undead than most ordinary humans. Taken together, tieflings should have average Charisma or a bonus. I suspect one reason the Cha penalty was left as-is was to preserve the symmetry of the format most races have of two ability score bonuses, one penalty.
Arguably, the modern tiefling should have average Intelligence, since they are less renowned as wizards in 3e interpretations and are not known for being particularly skillful. Nor are they known for their towering intellects. Is a tiefling as smart as a gray elf or a blue?
My take on the tiefling would probably be +2 Dex, -2 Wis, +2 Cha. Thus, they would suck as clerics, druids, and paladins, while excelling as rogues, bards, and sorcerers.