I have a three tattoos, and I love them all, and I'm a woman. One of them is on my upper back I can easily hide it, but sometimes I don't (the others are always hidden). In my experience, the lab is pretty much where we live, and during the normal work week (non TA days, etc.) there is nothing professional about the way any grad students dress. In any situation where I want to look professional, my wardrobe is not limited by my tattoos.. So, location is important, but don't let grad school stop you if you really want a tattoo, and don't feel like it necessarily always has to be covered. I do agree with avoiding neck/arms/wrists, or areas that can't be easily covered.
