Grid of evenly-spaced items using text-align: justify. No need to set spacing in between columns! Changing grid item width and bottom margin is the only thing you need to tweak inside your media queries to redefine columns-per-row for different breakpoints.

spaced_grid.scss

// container for evenly-spaced grid items
.l-spaced-grid {
  text-align: justify;
	font-size: 0;
	&:after {
		content: '';
		display: inline-block;
		width: 100%;
	}
}

// evenly-spaced grid items
.l-grid-item {
	font-size: 16px;
	@include inline-block; // compass inline-block mixin
	vertical-align: top;
	width: 18%; // desired column width
  margin-bottom: 2.25%; // spacing below grid items
  
  // just for demo purposes
  padding-bottom: 18%;
	background: #ccc;
}