NULL saves disk space?

[Phoenix Kiula <phoenix.kiula@xxxxxxxxx>]

Possibly a dumb question but there isn't much about this.

http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=postgresql+null+value+disk+space

I have some BOOLEAN columns. 90% of the cases of the columns is FALSE. Do I save disk space by having them as NULL instead of FALSE? So my application would have conditional code for NULL and TRUE, instead of FALSE and TRUE. 

Thanks...