As far as I can tell there was not a factory spec, there was a big thread on this subject on another forum and the general consensus was that they should be 1 - 1.5" out. If they are flush the valance gets sooted up pretty quickly and if more than 1.5" out they look like crap.
mine stick out less than half an inch, well known long time mopar shop in Denver Mopower masters just did my exhaust about a month ago and that is where they set the tips at. Tony has been around forever and is one of the most knowledgeable mopar guys around. your personal preference is all that really matters. if they look right and good to you. I have some photos of my car here under welcome wagon if you want to take a peek just search Golden.