I wouldn't use that on more than one door, ever. Because it's mostly water, it will refreeze quickly, so that by the time you get to your destination, your lock will be pretty much seized up.

It's my experience, driving in Canada, that it's somewhat rare for all the locks on your car to freeze. On several occasions, I've had to crawl into the driver's seat from the passenger side, and I suppose in a crisis, I could crawl into the front from the hatchback; but I can't recall ever being completely frozen out, even when I've been silly enough to run the car through the car wash without checking the weather forecast for the next day <img src="images/graemlins/blush.gif" alt="" />

Keeping a small bottle of lock de-icer in the glove compartment isn't such a silly idea, if your main concern is not being frozen out of your car, but avoiding the annoyance of having to crawl in and out through the passenger side until Spring springs. <img src="images/graemlins/cool.gif" alt="" />