If some people PERCEIVE an inoffensive, harmless tattoo that way, then so be it. Perception may be reality often times, but if the tattoo isn't offensive or overbearing, then its that person's problem.
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If someone THINKS it's an issue, than it's already too late. This isn't MySpace, where if someone doesn't like you, you just not on their "friends list." (How's that for relating to the younger crowd) If someone, superior or subordinate thinks a tattoo takes away from your credibility as an officer, then it already has. If you're will to take the hit, than so be it. I do concur that as long as it's not visible in PT, it probably won't be an issue--we aren't the Spartans, and we don't prepare for battle by wrestling naked. However, the more you get, the more likely it will be outed. 1 or 2 small ones with a school or service-related logo probably won't be an issue even if detected, but the more you get, the more likely you will be "that guy," and maybe won't even know it.
Also, I'm pretty sure the plural of tatoo is "tattoos," not "tattoo's." Plurals don't come with apostrophes.