A few more data points:
In "Dark Frontier", a photon torpedo detonated inside a Borg ship (after it was beamed over) and it lacked the characteristic glow as it built up to detonation.
Also, Voyager had differentiated their standard "Type 6" photon torpedoes from the high-yield "Type 10" photon torpedoes which were used on occasion. It might be starting in the movies that photon torpedoes went from standard large volume effect weapon to standard directed-blast weapons. Since we also know that warhead charges can be swapped out, not every photon torpedo would have antimatter as it's warhead. As Timo points out, this versatility and variability would make photon torpedoes hard to pin down... IMHO.