Can spar stresses be calculated using zero (wing) fuel?
In general, combining an empty wing with a full fuselage will give the highest stresses in the wing spar at positive and negative load factors. If the wing tanks held fuel, the weight of the fuel mass would reduce the wing spar bending moment inboard of the tanks at the same load factor.

However, details depend on the particular configuration. Make also sure you look at the most forward c.g. location and at the added load from aileron deflections.

Also, don't assume that the critical load case of one design will also be the critical of another. Please look at cases in all corners of the envelope to make sure you do catch the critical one.