The weight of the enlarger head can warp some baseboards, especially the cheap baseboards made of particle board. Shimming the bottom of the column sometimes suffices to correct the lack of parallelism between the baseboard and the rest of the enlarger. If not, it's easy to make a new baseboard from heavy plywood. As the OP hinted, it's important that film, lens mount, and baseboard (or easel) be parallel, but they don't need to be level.