“Stretching” is actually something of a misnomer, as it is unlikely that you are actually lengthening the metal of the string. What you are doing is tightening up the windings around the tuners, and pulling the string through any places that it might get caught, such as the tailpiece, bridge, saddles, or nut. If you pull on a freshly wound string, you will notice that it drops anywhere from a whole step to two steps flat on the first stretch. You need to keep pulling, over and over until it no longer goes flat at all. Do this every time you change strings—which you should do often—and you will find that you will stay in better tune, regardless of the quality or condition of your tuning machines.