It depends on what you are growing. From Orchidguy at GW:
As with anything in life, these are only averages. My plants never read the book!
Temperatures variances far outside these ranges can cause slower growth, flower drop and other problems.
Higher temperature greenhouse: tropicals, plants grown for their fruit (tomatoes, citrus, etc.) 60-85 degrees F.
Moderate to cool temperature greenhouse: houseplants, plants grown for their leaves or roots (cool season veggies), overwintering plants, maintaining shrubs, cool weather vegetables.
45-75 degrees F.
All greenhouses have 'micro climates', areas that are warmer or cooler than the thermostat says. These can be around windows, along the west wall in summer, etc. You can use these areas to your advantage, watch out for them!
In the real world, summer temperatures will exceed the optimum growing temps, for any type of greenhouse unless you have a very tightly controlled environment. Southern areas of the US may have problems controlling temperatures unless cooling systems are installed. Northern areas of the US may have the opposite.