even in a high wing plane, the lack of dihedral will slow it's tendency to self right because it will use only the pendulum effect of the CG being lower than the center of lift. dihedral accentuates this pendulum by allow air to slip off the wing tips until the lift on the high side equals the lift on the low side. that, is the whole purpose wings have dihedral. with a straight wing it is generally accepted that you have to use some opposite aileron input, to return the plane to level flight after a bank.