+1 for a rule.

> Always put a blank line before a _top level_ selector.

@grahamgilchrist If a file is considered a _contextual grouping_, and it has only one _top level selector_, there'll be no blank lines within the file and it'll become hard to read.

```sass
// _navigation.sass
// The entire file will be this: no grouping.
.top-level
    style
    style
    style
    .next-level
        style
        style
        style
    .other-level
        style
        style
        style
```