They are really cute, sorry I missed this when you first posted it .

From what I've read, tiels are hardier than most "tropical" bird species as they live in the wild in areas where it gets cooler at night than say parrots species who live in jungles. In the winter, we keep the furnace at 72 in the daytime and 70 at night (and I cover her cage at night with a blanket), and she seems to do fine.

By the way, I have found this site to be an EXCELLENT resource in regards to any cockatiel questions and info you might seek: http://www.cockatielcottage.net/

Under the FAQ part, there is information on ideal temps too (65-80), but warns that tiels do not do well with SUDDEN temperature fluctuations.