I realize this thread is kind of old but I wanted to say that tattoos are probably more common in grad school than most would expect. I have never had an issue with mine being visible in any way. I also kind of disagree with the poster who said it might hinder your chances when applying for jobs. I'm at a top private school and there are a number of professors with full sleeves here. I should add here that none of them actually wear short sleeve shirts when teaching but still, they walk around in tshirts in the summer when very few people are around. My personal view is that, while it's important (and totally cool) to be professional and keep them covered when teaching/taing, you shouldn't hesitate to "show them off" if you feel like it. In any case, no one really cares and those who judge you would find something else to judge about you even if you didn't have those tats.
