Well may growers do keep a constant temperature through out their grow. They have A/C running 24/7 and keep their room with in a 3 degree swing over 24 hours. The idea is to keep it cool, not cold but then again its fine to have temperature swings in the 15 deg range as well. The cooler temperatures in the dark cycle does help the plant to move around minerals and nutrients a bit easer.

68f lights off to 72f lights on is just about right if that is what you are thinking.

Hope that help some.