It depends on what you are growing the hemp for. If you need to flower the plants inside the greenhouse, then light deprivation is needed. This includes breeding and growing for flower. If you are only using the greenhouse to house mothers, cut clones, or start seeds, then light deprivation is not needed as these are all vegetative growth environments requiring 18 hours of light per day minimum.
Sometimes, permitting counties do not allow lights to run in a greenhouse when its dark out, so if you do need to maintain 18 hours of light in your greenhouse throughout the winter, then light deprivation may be necessary to hold light inside the greenhouse.