It depends on a few things, like insurance requirements, etc.
If you lose a bit Before, you will be healthier for surgery and lower the risk if complications.
I'm trying to retrain my brain about food before surgery. I want to get used to eating consciously, smaller portions and because I'm hungry, not because I'm experiencing emotions. I'm also trying I NEVER have seconds. It's just unhealthy. I'm also learning that it REALLY DIES take 15 minutes for your brain to realize your hunger is satisfied.
I think that I would be setting myself up for depression if I waited until after surgery to make any changes at all. It would be too rude of an awakening for me