This is one of those questions in medicine where the question never changes, but the answer does. Over the years I have seen the pendulum of advice swing back and forth from a shorter to a longer period of abstinence before an IUI.
Here's the latest. A study reported in the January 2010 edition of Fertility & Sterility shows a higher pregnancy rate when the period of abstinence before an IUI was less than 2 days. This shorter interval resulted in the highest pregnancy rates per cycle compared with longer intervals of ejaculatory abstinence. This higher conception rate occurred despite a lower total number of motile spermatozoa inseminated. This makes sense in view of our observation that a shorter interval may reduce DNA fragmentation in sperm.
Remember that it's best to follow your own doctor's advice in this regard because some men may have issues that may not make this practice ideal for them.