Once my surgery was scheduled, I kept it to immediate family, close friends, and those at work who needed to know. Your surgery is your decision, just like its your decision on who to tell and who "thinks" they need to know. Like others have said, I kept mine on the down low until after I was recovered and healthy. There are ways to sneak around the response of "i had WLS"...but at a certain point I felt it was too much work. I wasn't ashamed of having surgery. I look at it in a positive manner. It is a life long tool for me to use to stay healthy. Plus, people will speculate anyway if you tell them you've lost 70lbs in 10 weeks like I have lol.
So in short, it's your choice. Just like any other surgery, illness, etc. Don't feel obligated to tell anyone that you don't want to.
Good luck with your surgery and I wish you the best of health and recovery!