The following is my opinion, and strictly for use in my campaign. However, you may wish to note my reasons for my opinion, and apply them to your campaign. <-- (My attorney made me say this.)
Poison itself is not evil, it is just a tool, as a lockpick set is "just a tool". And, just like a lockpick set, the evil lies in its use. Use of a lethal poison is an inherently evil act. "Lethal" describes any poison, that in normal doses, can damage ability scores to 0. (No, it's not quite lethal, but making someone that helpless often leads to death.)
Of all the poisons mentioned by name in the DMG, Drow Sleep Poison can be used by Good characters. Finding it can be difficult, since only the Drow hold the secret. There may be other nonlethal poisons out there, but that's generally the purview of magic.
On the other hand, almost all countries in the world have outlawed the possession or use of poison. For this reason, even Sleep Poisons are considered unlawful. Since poison is a traditional way of ending the current ruler's reign, poisoners are generally considered to be assassins of some sort.
"Ravages" don't exist in my campaign. Period. Taking the tools of your enemy is the first step towards becoming him. Or something like that.
Again, these are my opinions. I'm sure you've got your own. When I play in your campaign, I'll live and die by yours. Until then, my players will live and die by mine.
Telas
Edited to add: In my opinion, the reason to keep non-evil PCs from poison are not to preserve game flavor, but more to preserve game balance. (Poison = higher CR, IIRC.) Allowing a PC to use poison makes him much more powerful, and transforms many fights into "duelling saves". I'm trying to avoid that if at all possible. Anyone who's had powerful wizards face off against each other knows what I'm talking about; the first to fail a save loses. :\
And, as DM, I would hate having to keep track of all the changes necessary when my group of bugbears, hobgoblins, and goblins gets hit by two different kinds of poison. For the PC, it's easy; he's got one character to worry about.
T