It's just easier to set up crossed in most cases. Fuse design has a lot to do with it, the way the fuse tapers usually makes up my mind for me. As DM showed, your always going to get some slack if you cross the wires. When I can I will run my wires parallel, if not I don't worry about it. I do spend a lot more time on my presision planes making sure my controls are set correctly without any slack but on a weekend sport plane if I have a bit of slack on the rudders pull/pull I'm not going to worry about it. You won't notice a thing.