Let me add this also. And maybe I shouldn't, but it' better to be informed and make your own decision rather than just listen to others tell you what to do.
In the electrical world everyone cheats everything. One instance was just last week. The boss said to run 350 MCM because he had some left over at the shop, but we really needed to run 500MCM. This was running on a ups system. I won't go into what that is, but it runs 24/7 and every computer in this area runs off of it, or at least their pwer is filtered through it. Was it correct? Not by a long shot. Will it set on fire? possibly. Would I be worried about it? No.
But thats a pretty big wire so they can usually handle running at 100% of their load instead of the 80% derating without too much notice.
In this instance, 14 guage is to run at no more than 12 amps derated. 15 is 100% which it is not sopposed to hit. But now you will ask that wire i your house to run 20 amps which is 133% of what it can handle. Likelyhood of a fire is very small. But a possibility. If your house burnt down and the fire inspector saw that, you wouldn't get 1 dime for your house.
So it's up to you wether the risk is worth it or not. In my house, I wouldn't do it.
Good luck whatever you decide!
p.s. Kade, I didn't mean to come across strong or belitteling (sp?) your knowledge. I read my post over agian and that's how it came accross to me, so sorry if i offended you.