We just returned from Roma and we go almost every year. It really depends but St. Peters does have people there all the time who check for exposed shoulders, shorts and, for the girls, low necklines or excessive clevage. Altho the rules are supposed to apply to both sexes, clearly they are more critical of women's clothing. This time particularly I saw young men with shorts about knee length who were allowed in but young girls with about the same shorts turned away. I also saw one young lady who came prepared with some sort of skirt (ladies, you will be able to give it a name) but she pulled it out of her pack, wrapped it around her waist and tied the cord, smiled at the guard and walked on in. He DID smile. Most of the other churches don't have guards at the doors so they are less likely to object. However, once inside the may see you and ask you to leave. I am not religeous at all but I do believe it is their church and so it is their right to set the policy.