I cannot disregard HR's policy, but most important is how your superviosr is. For sure, it would be a good idea to wear suit in the first week. At least you can give a better image to your new colleagues and also you don't know what they think, dress in suit is safe.
However, after 1-2 weeks you can "feel" what your supervisor want and whether it is appropriate for you to dress in suit in carrying out your duty.