Originally Posted by Daily Commute
I wasn't talking about falling sideways. I was talking about having your rear end flip over you. If you have weight on the rear tire, you can apply the front brake far more aggressively and stop much more quickly. If you have weight on the front tire, you must be much more gentle with the brakes. And in traffic, sometimes you need aggressive breaking.
Your rear end will not flip over you if you have a load on the front wheel. I have toured and commuted with front panniers only for a very long time and there is absolutely no issue with pitch-over over what you would expect on any bike. Even under aggressive braking conditions.
A heavily loaded rear wheel will result in the handling issues addressed by grolby, however. I find that even a lightly loaded rack bag will affect the steering, sometimes in very unpleasant ways.