Full-scale wind tunnel tests conducted in the early 1930s ago indicated that giving the upper wing slightly more incidence reduced induced drag very slightly. However, I find that bipes are better at aerobatic line holding with about one degree less incidence in the upper wing than the lower.

By the way, the old story that giving the upper wing more incidence (positive decalage) on a bipe with positive stagger makes the nose tend to drop before stall occurs was proven false during the same test series. Although the loss of lift of the upper wing would tend to lower the nose, the large increase in drag of a stalled upper wing tends to cause the nose to rise. The drag-produced pitch-up effect was found to exceed the pitch-down effect of loss of lift, causing the nose to tend to rise, instead of to fall, as the wing ventured into stall territory. Positive decalage tended to make this unhappy tendency worse.