why 12 h culture for plasmid prep? - (Jun/10/2011 )

I know that one should use an over night culture for plasmid DNA preparations. The culture is supposed to be 12-16 hours. I have never gotten any explanation of why not 10 or 18 hours?
Maybe the reason is very obvious but I don't see it. Could anyone enlighten me?

Too short an incubation to get a big amount of DNA, and by 18 hours the cells will usually have stopped propagating in a logarithmic fashion - so you get less plasmid yield respectively.

Thanks. I figured it was something like that but I thought that maybe there was more to it such as an effect on DNA if the cells were "too old".