Unfortunately, sheets of ply are rarely nice and true, especially in a length long enough for the keel. As John says, if the frames or keel are fixed down to the building board it will be pulled straight. 5mm in the middle of that length is no problem.
Again, as John says, fix some doublers either side of the keel (the plans may call for these anyway) and once the hull construction is complete it will retain it's trueness.