when they "wash" they sperm before insemination, it is a type of centrifugal wash. apparently the lighter, faster male sperm rise to the top and the heavier, slower female sperm are at the bottom of the wash, the last to be inserted, and therefore, since they are slower anyway, you are slightly more likely to have a boy with iui.
forgive me if this is an old wives tale or has been proved incorrect...nine years ago when i did my first and only iui this debate was kicking around.