The main file (usually labelled main.scss) should be the only Sass file from the whole code base not to begin with an underscore. This file should not contain anything but @import and comments.
Note: when using Eyeglass for distribution, it might be a fine idea to name this file index.scss rather than main.scss in order to stick to Eyeglass modules specifications. See #21 for reference.
Reference: Sass Guidelines > Architecture > Main file