NetGeek said:
What is the general rule on how far your speaker wires should be from AC power lines?
The greater the separation the better, but the 16" rule is generally accepted as a good one to follow (as JohnA points out) and is obviously convenient due to standard wood-frame construction techniques. The key however, is not to run your cables parallel to AC lines. If there is no way to avoid a parallel run, maximize the separation and keep the run as short as possible.
Where speaker cables (and other low voltage wiring) cross AC lines at 90 degrees, EMI will not occur. In other words, at any angle less than 90, some amount of EMI will occur with the value approaching zero at 90 (lets not do the math

). It is possible to use (properly grounded) shields to prevent interference, as you suggest, but in your case is absolutely not necessary.
It sounds like you've done a great job. Get it hooked up, turn it on, and enjoy.
I couldn't find spec's on the cable you used, but if it was wisted pair that will also help minimize the effects EMI. With twisted pair, the tighter the lay the better.