One technique (among many I'm sure) to ensure a straight trailing edge on control surfaces is to cleco a piece of angled aluminum (match drilled) to the trailing edge as the epoxy cures over several days. To make sure the angled aluminum does not epoxy itself to the trailing edge, a piece of heavy duty tape was stuck to the angled aluminum and the side going against the rudder was brushed with a mold release. After 3 days of epoxy curing the clecos were removed and the angled aluminum released very easily.
I'm a first time builder and no expert to be sure but I was taught this at Synergy Air which has used this technique over hundreds of builds of different RV models. The angled aluminum was match drilled many builds prior to my build, I just used it. This deviates from how the plans direct it be done, but a very straight trailing edge was achieved on the rudder and both elevators. You can see some pics of this process for the rudder on my blog dated 14 March.
Emp & Tailcone complete except fiberglass
Working on QB Wings