I don't follow any rules. I divide whenever is convenient for me. I try to watch the weather and make sure it won't interfere. For instance, the next ten days are going to be awful hot. So I don't do anything like that in times like these. But I have not found any significant difference between spring and late summer dividing. Do be careful though not to plant it too deep. If the crown is too deep the plant will not thrive. Made that mistake a few times. Some times I pot up my divisions and wait for them to bounce back before I plant them back out.
As a side note after some hard frosts I take the leaves and twist them up and form a "bun" over the crown to protect it from winter rot.
