That's standard for planar mapping ... To explain, think of a deck of cards that has a design printed on the back of the deck. The card is a flat plane. When you stack all of the cards up in a pile, the design on the back sort of repeats along the sides ... That is what is happening with your planar mapping. The plane is only a cross section that repeats from front to back, so you notice stretching on faces that don't lie along the same plane as the front. If you are using UV Mapper Pro, you can spread the outer edges of the model, and then relax the UV's so that they smooth out a bit. I think there are some tutorials on UV Mapper's site, or in the UV Mapper forum here.