A couple of notes about cable stretchers (aka "fourth hand"):

1. Almost never needed for Rear derailleurs. Just pull the cable tight with a pair of pliers. I pull them tight with my fingers all the time and just use a turn or two with the barrel adjuster.

2. Not normally needed with Front derailleurs, either. Usually, a pair of pliers will do. Just take the slack out with the pliers, lock down the cable, and then use the barrel adjusters to finish the tensioning.

3. Most brake cables don't need this tool either. Just squeeze the pads against the rims (fingers will usually do, but a third hand clamp helps sometimes), pull the slack out of the cable and lock down. Again, the barrel adjusters will finish the fine adjustment.

4. The one type of brakes that I find often needs the use of a cable stretcher is cantilevers. These seem to be the hardest brakes to adjust (and, most hated by LBS types). I love v-brakes ;-)

One tip that makes any and all of these cable adjustments easier is that any and all barrel adjusters should always be screwed in all the way (i.e. CW) before you lock down any cable. This gives the barrel adjusters the best chance to get the cable to the correct tension. If you can't get enough tension with the barrel adjuster, THEN it's time to break out the fourth hand cable stretcher.