I have a grid set up of thumbnail images, currently 4 thumbs per row. To make sure they line up i have this snippet of code:
li:nth-child(5) { margin-left: 0
You need to use variable interpolation to allow nth-child to be a variable.
$galleryGrid: 5;
li:nth-child(#{$galleryGrid}) { margin-left: 0;}
Generates
li:nth-child(5){margin-left:0}
This markup is fine if you have absolute control over the images and layout to ensure that your elements always wrap in such a way that every 5th one begins a new row. If you cannot make such guarantees, setting negative margins on the parent element is a better way to go.