I tend to add 6" borders, as that seems to look pleasing with the 12" blocks that I usually make. The 6" comes from the 6.5" ruler that I can cut the borders with. Just laziness for me. It is a pain to try to cut wider borders. I will also add a narrower inner border most times.
Of coarse, if I feel like doing something different, I will. This can be because of a lack of enough fabric.