@Ruth ******
I'd have to disagree that that is always how laws are worded. My experience with law and legal documents, including my divorce agreement, would never use the word "should" if it is a requirement. It completely leaves the law open to be challenged. Legal documents and laws are usually extremely specific and careful with the wording. If you search should definition in terms of legality it is clear that it means it is recommended, but not mandatory. Not saying that it is not a law, but the link you sent does not make that clear. Yet the link I attached clearly states that it is not a law. US customs and border protection also states that it "recommends", not that it is required. Although I can also find that they say it is required, at least for entering the US. Clear as mud.
Either way it is definitely something one should do out of common courtesy and I believe I am required per my divorce agreement anyway.