Aerodynamic theory says that total drag should be minimized by having both wings develop the same lift, for an equal-wing biplane. Since the lower wing flies in downwash produced by the upper wing, this would seem to dictate using more incidence on the lower wing, in order to offset the effect of downwash. However, in the early 1930s, when bipes were still the dominant airplane species and a lot of talented people were fine-tuning their aerodynamics, extensive full-scale wind tunnel testing revealed that total drag was actually very slightly lower with the upper wing set at about 1-1/2 degrees higher incidence than the lower wing. The reason for this is still, I think, unclear, but the effect on drag was so low that is was hard to measure.

The concept that having the upper wing stall first will produce a gentler stall was found to be untrue, at least in wind tunnel testing. The reason for this appears to be that when the upper wing stalls, the loss of lift is relatively small in compared to the great increase in drag, and although the loss of lift up forward should cause pitch-down, the sudden increase in drag acting well above the CG tends to cause a pitch-up moment that exceeds the pitch-down moment of
the stalled upper wing. The overall effect on pitch was not great.

Biplane lower wings were found to remain unstalled, due to the effect of downwash of the upper wing producing a sort of slot effect, up to more than 30 degrees positive angle of attack. This definitely does produce a gentler stall - the lower wing hangs in there, and its ailerons remain in business, well after the upper wing is in deep stall.

I too, find that my bipes perform better with the upper wing set at slightly lower incidence than the lower wing - maybe one degree or so at most. Aerobatic line holding seems to be improved.