My reason for saying no is different than everyone else's. So many dog bites happen because a kid approaches a dog sleeping on a couch, or the dog deems the couch their territory and will snap at a child if they approach. My first dog was a pushover and I had no worries about him around kids, but I still didn't let him on the couches. My current dog had resource guarding issues when I adopted him, so setting boundaries was really important.
That said, when I'm not home I know full well my dog lays on the couch. I can hear him jump off when I open the door.
You two beat me to it. We don't let our dogs on our furniture or our bed for exactly the above reasons. Our furniture is for the boss of the pack, the floor is for the dogs who are at the very, very bottom of the pack. Long, long ago, when we were young and stupid about dogs, we let our basset hound on the couch all the time. He was a powder puff and we never say any kind of aggression from him. Until the day a friend's toddler decided to park himself on the couch to watch some tv while the mom's were talking. Dear Basset Hound took exception to a stranger sitting on "his" couch and snapped and bit the boy. That was my introduction to the world of dog training. And yes, the boy was fine.I voted no. In terms of training a puppy, it’s best if you can make it very clear that you are the alpha dog in her pack. Letting the dog sit on the couch literally raises her place in the pack. The couch is for you, the alpha. Her place is close by, on the floor. These small things shift the power balance and you may get a dog that thinks she can try and be the alpha. Makes for a dog that does not obey and takes liberties in other areas.
We have a breed that continuously challenges for the alpha spot. We have to be firm and consistent in order for the dogs to be safe. We also do the sit/stay before going out a door, ie: they NEVER go out the door before us, we make them do a down stay before they get their food bowl, and many other dominance establishing routines.
OP, with a German Shepherd, who is intelligent and protective, the last thing you want is for your dog to think they are above you in the pack. That is a bite waiting to happen.