I bought my slow-build -8 kit third-hand and the two forward bulkheads were not very straight at all. However, when placed on a flat surface, they would lay flat with light hand pressure. I used them as they were, without any additional fluting. Mark the flange center lines as described in the instructions, and when it comes time to install the forward top skin, match drill the bulkheads starting from the longerons, working up, cleco-ing every other hole, or so. Adjust the bulkhead position by hand as you go, keeping the flange center lines visible through the holes. Much later in the build, when the top skin is riveted in place, the bulkheads will be straight forever.
RV-8 Fuselage Slow-Build In-work; on gear
Tail Kit complete