UV islands distorted? Use this little known but handy tool

It's essentially a "heat map" that will show where you might have some distortions or stretching in your UV islands. Stretching, of course, can effect how textures are applied to your mesh, and having Blender analyze your UV islands rather than just trying to eyeball things, or fiddle with things, is much, much easier. As always, Blender Guru explains it best. You can just skip to the 11:00 mark.