bleed for SUSY grids. Borrowed from: http://stackoverflow.com/questions/11223927/susy-how-to-extend-content-box-to-cover-grid-padding-as-well

bleed.scss

@mixin bleed($padding: $grid-padding, $sides: left right) {
  @if $sides == 'all' {
    margin: - $padding;
    padding: $padding;
  } @else {
    @each $side in $sides {
      margin-#{$side}: - $padding;
      padding-#{$side}: $padding;
    }
  }
}

Although susy 1.0.6 now has one of these built-in.