UV stretching during the unfolding process, a little is unavoidable even in the most meticulously mapped organic forms. The process of minimizing this can be a hair pulling experience. Also, the size between images projected onto each island/shell can be tweaked during the mapping process through various means in order to minimize the size difference. E.g. Wings3D/Scale/Normalize Sizes.

Creating textures for models with images is largely dependant on the model itself, as BB stated with his Colorado analogy. You have to use your judgement, experience with that model's UVs, and a lot of test rendering. Eventually, with practice, you'll arrive at a consistant texturing workflow for a particular model.