Job well done as it is a fact and part of the national code that a 20 amp circuit must be at least 12 gauge wire.
Theoretically, all building codes were designed to protect the next owner (or renter), not you.
Laws are meant to protect others from our actions, not to protect us from our own decisions.
If all you are ever going to do is plug in a light bulb, any wire will handle that current.
What about the next owner though? If they want to add an outlet, they shouldn't have to worry about whether you saved a buck by using 14/2 instead of 12/2.
The part that will be visible to them is the 20 amp breaker.
If the code is followed, the future owner will know that if they follow the same rules, there will be no safety concerns.
However, if you deviate from the code, the future owners could follow code in their new work and unknowingly create a dangerous situation through no fault of their own.