If you're like 90% of the population, you probably use your right hand. That is fine, except for the fact that your left hand is one that is then controlling the bike. And that means if you have to hit the brakes fast and hard in a quick emergency stop, you're going to be mashing down on your front brake only and can easily throw yourself right over the handlebars of the bike.
So, try to get in the habit of using your left hand when reaching for your water bottle, or fiddling with the zipper on your jacket, or whatever you might be doing when you're on your bike.
Just another bit of safety advice for you. It's a noble public service, I know, so please, no applause.
Source: About.com: Bicycling- David Fiedler