Ditto. Funnily enough you also get some rules that tell you to use id selectors:

    Overqualified Elements	Element (a#galleryLink) is overqualified, just use #galleryLink without element name.
    a#galleryLink {

This was given as a warning, but it's spot on too - https://developer.mozilla.org/en/Writing_Efficient_CSS for example.

I think it's actually saying a#foo is worse than #foo (assuming you're not reusing #foo in which case, conceptually, shouldn't it be a.foo?).

Indeed, I'd argue it ought to say that you EITHER ought to use a.foo or #foo depending on what you're doing in the page.

I'm not an efficiency nut, but it's quite possible #foo runs faster than .foo and this might be important to you, even if giving multiple things the same ID makes you feel dirty.