For many (not all) plants, more sun the better. The issue greenhouses have with the sun is the heat that it generates. You can hit 120F easily in a greenhouse in full sun even in northern climes. Shading means less sun which means less heat, but it also robs the plant of energy. Then you start getting into alternate methods for cooling, such as ventilation, swamp coolers, misters, heat exchangers.
Manually ventilating is tricky, be vigilant if you go this way, the days get longer, the sun moves - shading disappears.