I was just reading up on a few random caresheets and two of which i read stated 75 is fine for the cool side.. and the other said that 75 is good of night time drop, but like adam said and most of you concur its better to have constant temps and not have them drop. Here is the link to a couple of the caresheets... (this one states its ok for 70-80 range for night time) http://coloherp.org/careshts/snakes/ballpyth.php
http://www.kingsnake.com/ballpythonguide/