I'm seeing a lot of unwarranted (IMO) down-moderation lately, mostly on posts that express a minority opinion or that question a majority opinion (judging minority/majority based on discussion context). As a result, there are many posts ranked -1 or 0 that probably deserve higher scores.
I'd like to remind moderators that you're supposed to "Concentrate more on promoting than demoting". In the meantime, I'm going to spend all my mod points on posts that I think have been modded down unfairly. I encourage others to consider spending some mod points in this fashion as well.
I never said that more comments deserve a +5... in fact, there seem to be too many +5 comments IMO. Modding a comment down to +4 from +5 isn't really "overzealous". With regards to being objective, I believe that a choice has to be made by the moderator: if you question your own objectivity in response to a particular comment, then just don't moderate it. The strategy you described in your post (i.e., don't mod up if you can't be objective, try not to mod down unless the post is clearly off-topic) seems like a good one. However, when I see more than a few reasonably lucid and on-topic comments modded down to 0 or -1, I start to think that something funny's going on.