Rokeby,
All the wheels on the road are safe. Some are heavier than they need to be, and some transmit more NVH than they ought to. The number of spokes is a very minor point, but lacking small divisors in your symmetry is generally a good way to avoid resonant vibration at lower frequencies.
An evennumbered spoke count is a sign that designers and stylists came up with something, then told the engineers to make it work. An oddnumbered spoke count indicates the process went the other way around.
Further reading? Nah. The math behind NVH is boring, and since it's so far beyond the scope of the DIY'er, there's not much written on it.
