I know that in 1st edition, good characters were not allowed to use poison and poison use was strongly associated in the rules with evil alignment.
I'm not sure how 3rd edition treats it, largely because it seems to do its best to avoid the question.
For myself, using or not using poison is something that I associate strongly with lawfulness not goodness. That is to say, I associate it strongly with codes of honor. A lawful good person might not consider it fair play, unless he lived in a society with rules governing how and when it could be used. If he did, he probably was living in a lawful evil culture. My favorite example of this is from Dune, where they have a whole culture built arround the honorable art of treacherous death. They even call it 'Kanly', and treat it as some sort of chivalrous sport. In a culture like that, even a Paladin could concievably use poison (provided he had legitimate cause for war, warned his opponent in the formal manner, and gave his opponent sufficient time to negotiate some other solution, etc.). Another example might be a primitive jungle culture which for good reasons used poisons on its weapons when hunting game. In this culture a CG person would certainly not blink an eye about using poison in warfare, and a LG person would have the justification of tradiation.
But on the whole, you want to avoid opening that can of worms. Generally this question is asked by players looking for shortcuts, min/maxing, powergaming, and uninterested in roleplay. If the player intends to poison his foes, he better darn well expect that his foes will retalitate in kind.
The simplest way to address this is using poison to murder or assault someone is murder or assault and THOSE are clearly not good acts. Other than that, just make poison use difficulties balance with the advantages (no real world army used poison extensively prior to WWI, and since then its use has been very rare. For one thing, the legal customs of war designed to minimize casualties forbid it.)
But, speaking of opening cans of worms, is anyone other than me bothered by the fact that most poisons are 100% gauranteed not to be lethal? It seems like over compensation for 1st edition to me. Shouldn't poisons that do WIS, DEX, or STR damage have some chance of killing thier target? People bit by a Funnelweb or a Black Widow (much less the monstrous equivalents of these), or stung by a Sea Wasp ought to have some chance of dying. Instead, 5 poisons in 6 (as it were) are gauranteed to at worst leave the person in a coma for a day. If the world really worked like that, you should be able to argue that poison use was a legitimate tool of pacifists. Maybe excess damage ought to go to CON, or maybe the secondary damage of any poison ought to be +1 CON damage (in addition to other effects). Maybe there should be save fumbles - roll a 1 on a poison save and you are allergic, take +1d6 CON damage in addition to any other effects. Maybe all three. What do you think?