Uh, that should be a never. The NPC classes should be just that NPC's; and, thus, a tool for the DM to use.
Try having the players write a background for their characters. Using the background they write assign one or two skills that normally are not class skills as class skills and then give them half that many skill points per level to assign to the new class skill.
For example, say the player writes a background that he was a guard for a local muckity muck and learned diplomacy and bluff from their on again off again contact with other officials. Assign both as class skills and grant a bonus of one skill point per level so that they may advance those skills. If the point is not spent in those two skills they lose it.
My opinion. Which likely is worth what was paid for it.
That's what i meant, cept i was taking it a step further. Take a level of expert as your first level. Its your background. Now having siad that, I was completely joking.
