It is not generally considered rude to call family members or members of the wedding party for details about the wedding registry. Some people might be irritated at having to field those sorts of questions, but it's just part of the job of being a mother-of-the-bride or member of the wedding party. Those inserts are generally considered tacky. I'm not sure what part of the country they are acceptable in, but I suppose it's possible. Indeed it is also tacky to suggest on a wedding invitation that the presence of the guest or a charitable donation is the only gift necessary. That type of statement is usually given with good intentions, so I think that most people would overlook the tackiness, but still: gifts of any kind are a taboo topic on any official wedding-related document/invitation.