There is an 8px margin on my body element that I don't want to have there because it makes the navigation look a bit strange. I can set the margin-top to be 0 pixels, or set a negative 8 px margin on the navigation, which is better and why does Susy have this in the template anyway?
This is not actually related to Susy at all. Susy does not apply any 8px margin to the body, but most browsers do by default. You can use a reset ("@import "compass/reset";
" would be one way), or simply override the margin setting on the body by hand. I would not use negative margins to fix it, unless you really want 8px margins everywhere else.