Good point! In fact, I think I have read that tire pressures drop about a pound for every 10 degrees the air temp drops. So..If you filled them to 35psi when it was 90 degrees out, they would be 30 psi at 40 degrees. (Plus they always seem to loose a little over time as well)
Feel free to correct me if I have my numbers wrong, but that is what I seem to remember. Bottom line, your tires will always be inflated to a lower psi in winter if you don't actively check them and up the pressures monthly.
Edit: I guess I was right about the 1 psi per 10 degrees F...
see this....
http://www.tirerack.com/tires/tirete....jsp?techid=73
Jeff