The electrical system needs the neutral to be grounded at 1 and only 1 point.
If both your normal Service and your generator have grounded neutrals, when they are connected together you will have the neutral of the system grounded at 2 points, which is a no-no.
If your generator neutral is grounded, you need to switch the neutral; if your generator neutral is not grounded, you can allow it to be connected to the normal system's ground via the system's neutral.
