Posted By:
Jim ClarkeAgreed on many points. Re-backing on any card is a form of altering the card. I think on most 19th century cards if they are blank backs there should be some leniency. For that time period it was common to have the owners name on the card or a little writing on the back (normally in pencil 
). I think anything from the 40's and up with writing on the reverse should be more scrutinized.
Being a post card collector, I do not down grade the card for having writing on it or being postmarked. Actually I prefer a validation on the back. Same thing goes with residue on the back. If a 19th century card has "extra" paper on it from being pulled out of a scap book then that is OK by me. If there is paper loss on it then I buy accordingly. Many people know how to take off residue and excess paper along with pencil marks. But I'm sure that is another topic for another day.... JC