Having them too loose can lead to flutter and we all know that is all bad. Bob is correct about the performance difference between pull pull and direct control. Back a few years I was heavily involved in IMAC and one particular season I had my spot at the TAS all wrapped up ( decided not to go though ). The airplane I was flying was set up with pull pull and it flew well but I had a 12" piece of 1/4" music wite in the stab tube to correct CG. I decided to move the rudder servos into the tail so I could drop that ballast. I went from 3 servos to 2 and had more positive rudder control. Now fast forward to my current 50cc sport airplane, a 33% Laser that was originally designed for a G-62 and I was going to put in a DLE 55. The CG solution was to put both rudder and elevator on pull pull. The system works just fine but there are a few rules that I follow. The first is that I always use a nylon coated Kevlar cable. It is the lightest thing available and will never stretch. Second if that the cables must retain the same tension through the entire travel. I am currently bringing a 40% Extra and am saving the rudder servo setup for the last part in hopes that the servos can go in the tail. I also think that for scale appearance pull pull is a viable solution as well. My current Dalotel project is pull pull on the rudder but has a separate tiller in the fuse that a single pushrod from the servo drives. Either way the key is to have a free moving surface that is tight and centers well.