The actual "stretching" of the strings is (IMO) mostly getting them seated more firmly at the peg and in the bridge so they don't slip. I think the "yank" does this better than just letting it sit and meditate on the weariness of life. You still want the strutural flexibility of the string to remain so that they will play (vibrate) well and stay in tune. Treat your guitar rough (within reason); it likes it.