I've ridden plenty in jeans with buttons on the back pockets and I've never seen any scratches. Suede (or whatever that stuff is) seats, slick seats, seats that were just leather over wood, seats that were leather over padding, english saddles...no issues.
That being said: if you're riding in a saddle that you would be broken up over, if it were to get scratched, I would find a different pair of jeans to ride in. Better safe that sorry!
Is your saddle synthetic, or leather??
if its leather, it will scratch it a bit ( but not too much )
And if it synthetic then probably not much at all! Haha and if it does give any MINOR scratch it wont be notice able haha:)
ALSO it depends how high up your pocket it on your butt! Chances are your button might not even rub against the saddle ya know?