geared bikes - front brake -> left lever
fixed - front brake -> right lever.

on my fixed i only have one brake, and i turn over my left shoulder a lot more to stare down cars, signal, etc. than i look over my right shoulder. taking my hand off the right to signal right for 2 seconds is not that big a deal. and, i'm usually in the city, which means a lot more interaction with cars.

on a geared bike i'm probably on some suburban backroads and no matter what hand i signal with i still have a brake.