If you have low SP, voting on comment is pretty much useless anyway, unless other people also vote on the same comment. Anything with a payout under 0.02 STU goes to dust anyway. So if your vote doesn't give at least that much it's just a waste of SP.
And that is what I try to tell the new users. Even right now my vote is below $0.02 so I do not vote on comments unless they already have a small vote and I can put it over the dust level. It is a difficult thing for new users to grasp, especially if they are coming from another social site that values up votes on comments and see it as a means of showing support. A vote on a post is always a better choice to show support for a new user. Leave a comment on the post and vote the post.