Every collector has a different opinion about this. You get some who
follow the Overstreet guide literally and say that folding a fold-in does degrade the
quality. You get others who understand that it's hard to find a Mad
that hasn't had the backcover folded and they're willing to overlook
it. So it depends on the attitude of the buyer...
I wonder what Al Jaffee would say?
For the record, The Overstreet Guide has the following quote:
"#86-First Fold-In; commonly creased back covers makes this and later issues scarcer in Near Mint." That implies to me that any Mad that has the fold-in folded is an automatic Very Fine grade.
Overstreet Guide Grading Definitions
With that said, I open up this page to all you serious collectors out there who care to share an opinion on this issue.