Please don't be discouraged from reporting things that could be interpreted either way. I know that the gamification system seems to discourage that, but that's the fault of the system, not your reports. Let me quote what I recently said about this in terms of being invited to be a mod:
Your success rate doesn't actually matter all that much. If you're above 60%, we'll teach you the rest once you become a mod. As long as you can maintain that, quantity of reports is much more important than quality. Specifically, we're looking for people who report things during time periods where the reports aren't immediately acted on, because that indicates a need for additional moderators for that time block. People who report things during those time are far more likely to be chosen, and you're more likely to report during those times if you report more in general.