Tomatoes are caulescent, one word that covers their natural habit of shedding early leaves as new ones are produced. The plant naturally closes down these older leaves, which takes energy from new growth and ripening fruit, so if you take the lower leaves off (snap them at the abcission layer) you are helping the plants. Assuming the plants aren't etiolated, only the leaves within the first four feet of the growing point are actively contributing to the plant while any others will be in energy equilibrium or taking food from the plant.
Taking off the leaves in a greenhouse is also beneficial as it will increase air movement and help reduce fungal diseases.
