A woman I worked with while in school had a tattoo behind her ear that ran down the side of her neck. The only people that really found it unprofessional were the patients on the geriatric ward and even so it was only 2-3 of those patients that ever said anything. Other than that no one had a problem with it. Is it the hospital saying it's "unprofessional" if so than yes get a t-shirt to appease them. Nike has some nice form fitting t-shirts with their moisture wicking fabric that would probably do the trick. If it's not the hospital imposing on you I'd just wear the tat proudly. I thought with my many piercings places might have a problem or deem it "unprofessional" as well, but that just turned out to be my own personal hang-up. Most of the comments I recieve about my piercings are out of curiosity, people wondering the stories behind them.