Spells Only Do What They Say They Do
While "[i]n general, positive energy heals the living and hurts undead creatures" (PH 311), some rare positive energy effects that inflict damage don't also heal the living. For example, the 6th-level Clr spell bolt of glory [evoc] (SpC 35-5) launches a "white bolt of hissing positive energy" that can only inflict damage.

It's best to adjudicate whether a positive energy spell that inflicts damage also heals damage based on the spell's description rather than making that in general a constant.

Effects typically do what they say they do

While "[i]n general, positive energy heals the living and hurts undead creatures" (Player's Handbook 311), some rare positive energy effects that deal damage don't also heal the living. 

For example, the 6th-level cleric spell bolt of glory [evoc] (Spell Compendium 35-5) launches a "white bolt of hissing positive energy" that only deals damage, and the 5th-level psion/wilder power celestial conduit [psychokinesis] (Complete Psionic 80) brings forth a "massive charge of positive energy from your psyche into the material world" that also only deals damage (although the effect does deal even more damage to undead creatures).

It's best to adjudicate whether a positive energy effect that deals damage also cures damage based on the spell's description rather than making that in general a constant.