When you start sanding you will have to sand enough to take any wobble or variation in thickness out of the hull. You don't want to leave variations in thickness between strips because it will pull the glass away from the surface over the step between strips. And remember that if your strips are out of alignment you will wind up sanding them smooth on both the inside and outside of the hull. This may cause a substantial reduction in hull thickness.

Ideally, you wouldn't have to sand at all and the strips would stay at 1/4 inch. Realistically, you will have to sand enough to remove variations in strip thickness and to fair the curves of the hull. You will probably wind up with some areas that are a full 1/4 inch thich and others that are somewhat thinner. The more consistently you can cut your strips and more evenly you can match the edges when you glue up the hull the less sanding you'll have to do later.