I run front=left on my two-brake bikes and front=right on my one-brake bike. I got an XT caliper, right lever, and 8" rotor for $40, so I just went with it on my fixed gear 29er/cruiser. If there's only one brake lever, you're going to find it whether it's on the left or the right (and if it's an 8" hydraulic disc, you aren't going to need much hand strength).

I also think that the safest "safety brake" is a brake you use. Every bike brakes differently, and the more accustomed you are to your brake(s), the more effectively you're going to deploy it (them) on instinct. If you're never used your brake before and a dump truck pulls out in front of you, you may not even remember it's there (and don't get me started on stem-mounted/seatpost-mounted/top tube-mounted brakes). Bikes don't have ABS, so stopping yours in the minimum distance takes practice.