One would have to do a series of micro- and macro-benchmarks to determine the exact benefits of enabling I586_CPU, I686_CPU, and I586_CPU with I686_CPU, on various CPUs, to see what the optimal setting is. There were a bunch of benchmarks done back in 2005/2006 with various encryption algorithms that showed enabling I586_CPU (with I686_CPU) on P3 CPUs gave better performance than just I686_CPU (due to all the bzero, bcopy, mmx copy, and other memory tricks that are enabled with I586_CPU but not with I686_CPU).
Whether or not that still holds true today, with Athlon64s, Core/Core2, Opterons, etc, I don't know. But there hasn't been anything since then saying it hurts things.