MTF pg. 165 says:
Soul Trapping.
Mortal creatures slain by hellfire engines are doomed to join the infernal legions in mere hours unless powerful magic-wielders intervene on their behalf.
Pg 225 of BGitA says:
A creature might have been imprisoned as a result of defaulting on a deal, while another might be the victim of a night hag's curse.
Pg 226 of BGitA says:
Hellish Currency.
Soul coins are created by Mammon and his greater devils on Minauros, the third layer of the Nine Hells, in a vast chamber where the captured souls of evil mortals are bound into the coins.
Here's is what I originally said:
"I would say that the Nine Hells is LE. Its lawful so are it denizens for the most part, So I think that they would be selective in the souls they corrupt and usurp."
I will admit technically I think we are both right. The OP is asking about soul coins which appear in a FR adventure, they didn't say whether they are playing in a Forgotten Realms campaign or playing BGitA, but it's possible. If we put this into a FR context here's my opinion. Kelemvor (LN) Lord of the Dead, Judge of the Damned, judges dead souls where one of two things happen, they either travel to plane of the god they worshiped in life or they are deemed faithless if they worshiped no god and wander the Fugue Plane. Hellfire engines can destroy any mortal whose soul is then forced into the ranks of the Devils armies. I don't see a whole lot of mortals going to the Nine Hells let alone good ones, unless its for a very specific purpose. So, a soul coin created with a good soul is probably the exception, although not impossible. I could see some deities intervening on a good souls behalf to save them from that fate. My opinion, backed up by BGitA says that soul coins are made from evil souls, I believe that the majority of soul coins encountered contain an evil one.