The main explicit reference to this is a note given in the Maeglin materials in History of Middle-earth 11, which was added to the text by Tolkien at some time after writing it, but which was not included in the published Silmarillion:
For the Eldar never used any poison, not even against their most cruel enemies, beast, ork, or man; and they were filled with shame and horror that Eol should have meditated this evil deed.
Use of the variant spelling "ork" dates this note to either the late 1950s/early 1960s or the late 1960s, since they appear to have been the main times when Tolkien used this spelling.
There is no indication for why this note was not included in the Silmarillion, although we can suppose that it might have been on account of the used of poison by the Elves of Nargathrond. (Although as Tim Pederick correctly notes, this need not be seen as a contradiction because the Elves of Nargothrond are stated to have "fell from the valour and freedom of the Elves of old, and their land was darkened".)
The second reference is note 9 to the Druedain material in Unfinished Tales, concerning use of poison by the Druedain:
They had a law against the use of all poisons for the hurt of any living creatures, even those who had done them injury – save only Orcs, whose poisoned darts they countered with others more deadly.
This may seem to contradict statements in Return of the King, but if you actually re-read the passage you'll see that it need not:
Let us be thankful that they are not hunting us: for they use poisoned arrows, it is said, and they are woodcrafty beyond compare.
First of all this is not Tolkien making the statement, it is Elfhelm; one must accept that Elfhelm's statement may be wrong. Secondly, Elfhelm is not saying that they use poisoned arrows; he is saying that it is said that they use poisoned arrows.
The third statement is in Letter 153 in The Letters of JRR Tolkien, which is a draft letter. It's not clear if a letter based on this draft was ever sent, and if so whether or not it included this statement (or something similar).
I should regard them as no more wicked or foolish (but in much the same peril) as Catholics engaged in certain kinds of physical research (e.g. those producing, if only as by-products, poisonous gases and explosives): things not necessarily evil, but which, things being as they are, and the nature and motives of the economic masters who provide all the means for their work being as they are, are pretty certain to serve evil ends.
This makes it clear what Tolkien's own opinion is: poison is necessarily not in itself evil, but is almost certain to be used for evil purposes.
There is no other statement that I am aware of regarding the morality of using poison, but the majority of other references to poison in Tolkien's work are related to deeds, works or creatures of Melkor and of Sauron, or to independent evil agents (Ungoliant, Shelob).
Taking it all together we can state with a high degree of confidence that YES, use of poison was considered to be immoral at best, evil at worst.