Has he ever not voted with the rest of the Democrats?
Not really. In 11, he got a 15 from the ACU (same as Menendez - NJ, Bill Nelson - Fl; less than Ben Nelson - Ne, Lieberman - Ct, Pryor - Ar).
His votes with the ACU were against greenhouse regulations (coal), against ethanol subsidies (coal), and for selling aircraft to Taiwain to increse its defense.
Only when he can...i.e. when leadership can allow a few to "vote their conscience" and still win (or at least not lose)