Some trim does have a very thin edge where it meets the wall. Take rubber cove for instance.
What I have done with this base cove is place tape on the rubber material and let it stick past the top of the base in the air if possible. I then took a 5-n-1 or mud knife and push the tape down to the intersection of the wall and base where it sticks. It does not have to be really straight. The excess tape on the wall is trimmed away with a very sharp utility knife. The knife blade rides on top of the base or where it meets the wall, hopefully giving you a straight edge. Painted with a not too heavily loaded brush until covered. Remove tape.