I suppose I should clarify Sass Guidelines to explain the difference between a string intended to be use as a CSS value, and a string when sticking to the data type only.
For instance, we do not quote sans-serif even if it a string, because it will fail when output in CSS if wrapped in quotes. For the same reason, we do not quote min-width because when printed as part as a @media directive, it will fail as well if quoted.
I hope it makes sense. I’ll add something about this to the guidelines, thanks for noticing.
Edit: done. This will be live in 1.0.1.