I used to use a 2em top padding. The reason for the padding instead of a top margin is that it works better when the space is at the bottom/top of the page.
But now I changed that and I use a 20% line for note a section break.
Code:
hr {
margin-top: 1em;
margin-right: 40%;
margin-bottom: 0.9em;
margin-left: 40%;
border-top: 2px solid;
}