You'll probably get nearly as many answers about convergence as replies to this post, for most people it's a matter or preference and flying style. It also depends a lot on the plane you're flying.
My guess most range in the guns on the American planes around 300-400 although I've known plenty of people that are shorter in and even further out. The .50 cal on most those planes has excellent ballistics in the game and still hits pretty hard at those ranges. A good burst can still damage outside of that but it starts to drop off. I think most people who fly cannon armed planes probably set there's a little closer because generally speaking the cannons can be a little harder to hit with because of poorer ballistics (generally) and lower ROF, and having less ammo on board to mess around with. The upside is of course when they do hit it's a lot harder than a simple MG round.
