Maybe it's personal preference and discomfort, but I always thought that wearing anything with "EMT", "EMS", "Paramedic" or a star of life off-duty opens you up to huge risk. This may not be a problem for you, but wearing it in public gives you a duty to act in most cases-- if someone sees you, they may mistake you as being in uniform, and they are likely to think you are trained to help, and assume you will help, when, in most cases, you cant help.
There is nothing legally wrong with it, T-shirts can say whatever they want, no certification required.
It's worth noting that most state patches and I know the NREMT patch is a trademark. You can only display it with authorization (c-card). On the NREMT website, you need to verify your certification before they let you buy anything.
Note: The NREMT will only fill orders for items ordered by EMTs who are properly certified at the level specified on the selected items.
I personally am very careful about what I wear off-duty. As soon as my shift is over, I take off my duty shift. I dont want anyone to see me driving my POV and assume I have an ambulance in the back of my car.
I do wear EMS clothing off-duty when I am on my college campus. Almost everyone who knows me knows that I am an EMT, and even if I see or encounter something, I have a duty to act on campus. Last night, in fact, a friend called me over asking for a curbside consult, which I generally do not do (ever, but esp when in uniform). I was on call at the time, so came over, but immediately paged out the rest of the crew. The patient ended up getting ALS'ed to the hospital, and I am glad I didnt hesitate to act.
Here's where I see the difference. Nobody is going to stop and insist on treatment from someone in an Army t-shift. What can a bystander expect from someone in a fire department t-shirt if they clearly arent with an engine?
Just my $.02
Good Luck!
DES