Unfortunately, the answer to your question is a little murky. First, there is no governmental authority with which you need to register. However, the Pensylvania Marriage Law requires that the officiant be clergy of a "regularly established church or congregation." As a result, some counties have refused to accept marriages performed by clergy who are not deemed to be bona fide.
In 2008, the ACLU of Pennsylvania sucessfully litigated several of these cases, obtaining lower court rulings that, to perform a marriage, clergy need not regularly preach in a church or have a congregation. However, because none of those cases were appealed, there is no controlling legal authority in Pennsylvania. As a result, some counties persist in refusing to recognize marriages if they are not performed by traditional clergy.
I understand that Philadelphia is very lenient in this area and that it will accept marriages performed by clergy of Internet churches, such as the Universal Life Church. However, it is possible that, somewhere down the line, a court may deem marriages of this type invalid.