The neg. incidence in the top is so the bottom wing stalls before the top one on landings or spin entries. This turns the plane into a high wing trainer just before the moment of full stall. This feature remains the same when it's inverted, the top wing (now located as the lowest wing) will now have + 1 degrees in it, allowing it to stall first and leave you flying off the highest wing just before full stall. This is a safety feature for full scale and for sport pilots. However this feature may not be favorable for all attitudes of the plane. Kitting companies put this in the design just incase the builder is slighty of in the construction of the plane. First flights are protected. First landings are protected from tip stall also. For Imac Basic I'd suggest following this, and it won't hurt your flight. As you move up in class, you'll want to have both wings at zero zero.
My Carden Cap flew with 1.0 positive inc. in the stabs. I had three of these, they all needed +1.0 in the stab to get the elevators to line up with the stabs once it's trimmed out. Don't put this in your bipe just because you read it here. Fly and see where the elevators are resting after you like your thrust (engine thrust) and CG. Then move the stabs to line up with the elevators if need.