A Bronze-rated PSU at 50% load achieves 85% efficiency, which is about the same as a Silver or Gold at or near 100% load. The difference is, the Bronze costs less and will probably last longer if ran conservatively. Mining places a continuous load (as in "24/7") on the power supply and graphics cards, which is not the same as "average computer use". So having an adequate power supply is a must, and a little overkill is recommended.
Also if you're running at only 50%-70% load, you don't have to worry about whether the PSU is single-rail or multi-rail so much, as you won't be putting stress on it. You don't have to buy a Gold-rated power supply to be efficient. They are nice though.