So I just saw a question that asked for a recommendation. These questions are almost always closed because they require opinions. Is it a good idea to comment and tell the OP they may want to delete it before it gets downvoted as an opinion based question?
You can always explain to the OP that the post is not suitable and explain why (links to related questions/blog posts are always a bonus on these).
With luck, the OP will delete the post themselves, saving the community some time and effort.
If the post gets deleted, the reputation lost gets refunded, so that's not a real consideration here.
In short - it doesn't hurt, but you shouldn't stop with just a comment. Flagging the post as off-topic is the most correct thing to do.