I only wore a belt when I was first home from surgery. My stoma is close to my incision site and I couldn't seal that side, so I used the belt for some security.
I do use a wafer with the most convexity you can get and I don't wear a belt now. I haven't had a problem with it falling off or leaking.. Using a belt or not is one of those personal things...you have to be comfortable with your situation and no one is the same.