What mods that should do tune?
For simplicity, let's start a list of those that DO require retuning.
Most engine mods that produce additional horsepower (heads, cams, aggressive MAFs)
rear differential gear ratio changes
torque convertors (not all, but some)
tire diameter changes (may affect the speedo)
some intakes (air flow ratio changes)
I'm sure people can add others to this list.
In short, most things that change the drive train may require some degree of retuning.
Most other mods do NOT require retuning -- that being said, even a stock, unmodded car can gain hp/tq with a retuning. The stock tune is normally for EPA emissions compliance, not for performance.
Hope this helps.

