I usually do it like this: make a block of masonite or similar that is as thick as the biggest gap. Trace the floor contour onto the trim with a pencil and use the block as a sliding support. Sand the underside of the trim up to the line at a slight angle using a one-handed angle grinder with a flap disc.
The problem with a dip in the floor is, as mentioned earlier, that if you fit the trim 100%, it will be lower than the trims it connects against.