Your approach should work fine for cool weather crops such as lettuce, cole crops, sweet peas, etc. But warm weather seedlings will not fare very well if temperatures drop much below 50 degrees for extended periods of time. When the temperatures inside the greenhouse begin to hit over 70 degrees during the day and drop down no lower than the mid forties at night then I would expect heat loving seedlings to do o.k. left overnight.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.