I use pro-res 422 LT if it is something with text that needs to be very clear and readable or things that are going to have a close more critical viewing location. Other than that pro-res proxy typically works fine and you can't really tell the difference. You will be able to run MANY more layers in proxy then with AIC in any testing that I have done, so I prefer to use it if I can.
We have some mac minis that we can run 6 layers of 4352 x 720 without a frame drop at 30 fps in pro-res proxy, so mac pros will smoke…