I don't remember who told me this, but another quilter once told me to create borders that are divisible into the block size. Such as, if you use 12" blocks use borders that could divide into 12 like 1", 2", 3", 4", or 6". Or, if you have 9" borders use 1", 3", or 4.5" borders. I don't always stick to this method, but it does work out fine when I do and the borders always look good.