No "rule" I know of but any new bike I've ever ridden was set up front/left and rear/right. I prefer that setup mainly because the front brake has the majority of the stopping power on wheeled vehicles and I'm right handed. If I have to take a hand off the bars to do something it would be my right and I can still have the front brake covered. Plus with my right being the dominant hand I'm (ever so slightly) less likely to overpower the front brake on loose surfaces using my left hand. Might all just be in my head but that's "why" I prefer it that way.