Experience has taught me that you're more likely to fishtail emergency braking with the rear than endoing emergency braking with the front, unless you're leaning over the bars.

I keep mine left/front, right/rear mostly because I'm lazy. I've considered switching it, though, as I do impulse brake with my right, and I do all my hand signals with the left (as it is closer to rearward traffic LOS).

M.